I do an enormous amount of container gardening because I find that plants in containers well up off the ground have the best chance of surviving my rather incredible browser pressure from nibblers (mice, voles, gophers, rats, rabbits, deer, triceratops for all I know). For containers I use whatever is cheap or free, which tends to mean plastic of various kinds. Here's what I wrote in another thread:
I am holding onto every 1.75l plastic whiskey bottle, every laundry detergent bucket and bottle, and every half-gallon and gallon food container that I can get my paws on. When I am feeling flush I buy a few $2.57 5-gallon plastic buckets at Walmart. But the demand seems infinite...
My best scrounging strategy is watching the roadsides on both busy highways and bumpy gravel side roads. The oil industry is big around here and they have an infinite fleet of work trucks that goes around servicing oilfield equipment. Apparently they consume a lot of lube and hydraulic fluid, and the empty buckets rattle off their rigs with regularity. Sometimes I get bigger drums this way as well, although I suspect those are often deliberately dumped. I do have to be picky about avoiding anything with residual contents that might be toxic or can't be safely washed out, but I have a professional background of dealing with hazardous site contamination so I've got a pretty good idea what's actually dangerous and what goes away with exposure to soap, water, bacteria, and time.
More practical in the long run is going to garage sales. Sometimes there I find actual flower pots for small money (today I got a stack of 50 starting pots in sizes ranging from pint to 3-gallon for two bucks) but the item that I'm most likely to find is old coolers, at prices ranging from a buck to about three bucks (my limit, although the asking price for old coolers that are still nice is often more than that). From 1.5 gallon drink coolers to 64-quart square Coleman jobs to 10-gallon beverage tubs, these all make excellent planters once you drill them out. I find the insulation helps moderate the soil temperatures, so my plants suffer less heat stress and need watering less often. Plus, they are sturdy and last for years. Often the garage sale items have broken lids, cracked interiors, or general dirtiness that make them cheap, yet still perfect for making planters out of. I sometimes also find larger metal cook pots for small money, or decorative wooden kegs, or the wooden bucket that used to be part of an ice cream maker, or... you get the idea. Basically I wander through garage sales looking for any vessel that (1) holds a gallon or more, (2) is durable enough to survive exposure to the elements for a couple of years at least, and (3) is cheaper or better in some way than a Walmart bucket.
For small plants I use a lot of disposable plastic drinking cups, although these typically shatter from sun exposure after one season; using these has become much easier since I saw the tip on one of Ken Peavy's seed starting blog posts to use a utility knive to shave the bottom edges of the cup in three places to create drainage slots. (Before that I was shoving a screwdriver through a stack of cups repeatedly; this was a lot more work and the resulting drainage was barely adequate.) There are really cheapy thin plastic starting pots at various dollar stores around here in short stacks with drainage holes already in place, but like the disposable cups, they tend to shatter after one season of sun exposure, which makes a mess, is annoying, and does not meet my basic preference for plastics use (which is to limit it, when I can, to plastic items that were heading for the waste stream before I got my paws on them.)
I really like working with coolers and other insulated containers; they tend to last longer, be made of heavier plastic, and they mitigate one of the biggest problems with container gardening, which is solar heating that can cook the roots of your plants or at least bake off all the soil moisture. So last fall I was at a garage sale when I saw a cardboard box marked "free!" that was full of those one-liter snap-lid double-walled insulated hospital water cups that they issue one-to-a-patient in every patient room. The garage sale had the feel of an estate sale so I was fairly sure these were mementos of some deceased person's depressing final eight visits to hospital. "Free" was the right price -- who wants to drink out of a dead person's cup? -- but I was making another purchase so I didn't feel bad about snagging the whole box of free cups.
Today I got out my battery drill and added the drainage holes:
You can see that a few of the drainage holes have triangular fractures near them, which happens when the drill bit catches going through the new hole. Converting as many plastic items as I do to use as planters, I've learned that there is a fine art to drilling plastic, which I have to relearn with every new project given how much the quality of plastics varies from item to item. Basically, the trick is to adjust the torque on your drill as low as it will go (my setting today was "4" on a scale of 1-15) and still get the job done. Even then, plastic is easy to drill and dropping a fast drill through a cold hole will still get you shatters. So the trick is to apply a very gentle pressure and wobble the drill-motor to and fro. The idea is you want to deliberately take about ten times longer than normal to complete the hole, letting the drill bit warm the plastic as you go so that it will cut cleanly (because it's slightly soft) rather than shattering (because it's still cold and brittle). In extreme cases if you get this wrong the drill will shatter your item so badly that the bottom falls out in triangular chunks; usually but not always, that's a sign of too much sun exposure before you got the item. With care, and taking it slow, you can drill most items full of drainage holes without problems.
I'm pleased about these hospital cups because they are twice as deep as most of the similarly-sized small pots that I have; that will make them especially suited to starting fruit tree seedlings, which I do a lot of. And being double-walled, they'll have some insulating qualities, although not as much now that I've drilled big honking drainage holes that disrupt the former integrity of the insulating captured airspace between the double walls.
Moral: keep your eyes open at garage sales. Even the most amazingly worthless crap can sometimes be repurposed!
I usually try drilling plastic with the bit in reverse, letting it mostly melt its way through. Slower, but for me tends to result in a better hole which isn't as likely to end with cracks spreading from it.
At my parents house, a lot of juice in fairly tall, narrow 1.36L clear plastic bottles is consumed. I've been saving these, chopping the tops off, and using them for cuttings, so I can see if roots have formed without removing the plant. Not completely optimal because the containers are slightly curved inwards, and have a bit of ribbing, which makes extraction more difficult... but the transparency is handy.
In general I am transitioning these days away from using accessioning new plastic containers for my garden; I do make exceptions, but they tend to be exceptionally thick or made of plastics I recognize as types that are UV stable and unlikely to break down into shards. (Select insulated coolers, blue food-grade barrels, or those indestructible green frog-shaped kids' sandboxes, which make exceptionally good and durable small ponds.)
Over the years I have spent too much time picking up triangular shards of plastic, because, predictably, my "quick fix" plastic containers have deteriorated in the sunshine and shattered. I have no regrets; when I was desperate for containers, I took what I could find and afford. But over time I've accumulated plenty enough metal, wood, pottery, and UV-stable plastic containers, and that's where I focus my ongoing acquisitiveness.
All of this is by way of introducing my update to this old thread. These plastic hospital water cups were a terrible mistake! Of all the cheap/thin plastic containers I've used over the years, these were the absolute worse for coming apart rapidly in tiny shards when exposed to the elements. They were even worse than the Solo-cup type single-use plastic cups, which shatter into larger pieces when they break.
Do not use these. These are anti-useful. They are just waiting to sit in the sunshine for a few months so they can explode into tiny triangles of translucent garbage. -2/10, very bad doggos, do not pet.