I would like to put a plug in for a company I've been buying from for a long time, and I forgot how much I like them. Bucks Run Sports. They can't get a website together, buy by phone only, and they have more than you would imagine. For some reason I just like how old school they are. It's been the same guy for over the 10 years I've been buying from them, I think the guy's name is Kip.
They are probably the single best source for shotgun hulls that I know of. Just ask, they have more there than they even know. Just yesterday I was asking about 2 3/4" plastic base buckshot hulls. He didn't think they had anything. A quick look found that they had thousands. If anyone is looking for some, they are selling them cheap.