Buy piglets from pigs that are being raised the way you intend to raise them. If you want to put them on pasture, buy from a person who pastures their pigs.
Beware of cheap piglets. You get what you pay for. One of my neighbors bought a couple piglets cheap from an ad on Craigslist. They wouldn't eat hardly anything that he provided them. They failed to thrive and to make matters worse they tasted terrible. At least that's the story he told me. He ended up selling the meat to friend for dog food.
If your fencing and shelter is not ready, don't get them yet. You get better deals when you buy more, but only get what you can handle with good conditions for the pigs Some people try to wean off early which can, but not always, cause issues. Look for heritage breeds for hardiness, tamworth are a good example that can handle weather well.
If you can find a breed association close they can point you to breeders. Also check out livestock conservancy for breeders in your area.
I have had success on craigslist but I was looking for specific breeds
It's feeding time! Give me the food you were going to give to this tiny ad: