Scrollmaster
Of course, the king of them all is Dixie Gun Works - if they don't have it it may not be worth having. Midway is cheaper for the most common things, though.
There are actually a lot of internet sources, you have to look for them. Look for Civil War re-enactment suppliers, they will have most anything you need as well.
I concur that it is best to buy powder and caps in person. Unless you shoot a whole lot, it isn't worth buying by the case. The shipping charges for dangerous materials makes it not worth shipping one or two cans at a time. It might be worth looking for one or two other guys with whom you might split a case, though. Then it's worth looking at Maine Powder House or one of their compeitiors.
I recommend the snail type (Ted Cash in brass) capper. The in-line that I have only holds nine caps, so I have to reload it every time I cap five. The snail type holds 100 caps, a whole tin. Powder flasks are of several types, I use the brass cylinder type because they are heavy, hold enough powder, and I can't bend them. Not historically accurate, though.
Good luck finding what you need.