rareseeds.com sells some things in bulk. I don't know how reasonable their bulk prices are or if what they consider bulk is enough for your needs though. what sort of seeds are you looking for and what quantities?
Fedco out of Maine does bulk seeds and volume discounts based on $$ spent. They carry lots of organics, and some biodynamic grown seeds. The have several different catalogs. The seed catalog is closed for the season till December. The organic growers supply is still shipping cover crops. Every year I get together with a dozen or so people to pool our order together to get 20%-22% off our total. The seeds are high quality and they offer lots of short season and cold tolerant crops.