Powder and primers I get locally, usually at Sportsman's Warehouse although I'll occasionally get these items at a gunshow.
I use cast bullets so I typically order those from the manufacturer. I've bought from Moyer's, Friendswood, Missouri, Dardas, and probably one or two others I can't think of at the moment.
Brass is typically once fired from Gunbroker or gunshows. On the rare occasion that i buy factory ammo I always save the brass.
GH1