I buy the subs locally, usually at WalMart's end of season sale. But most of my shooting is with GOEX which I get by the case from Jack's Powder Keg.
http://jackspowderkeg.com/ I pick it up from the proprietor when he attends reenactments in my area. The problem with Jack's is that you have to by a case to get it shipped. That may be a lifetime supply for some shooters. But heck, it doesn't go bad and it's not going to be cheaper as the years go by.