I've had luck on ebay.ca, of all places. I was just looking the other day and it was dry but watch it often because there are a couple "stores" that occasionally part out a rifle or two, a variety of models. The bidding on the parts though can go crazy. I've also picked up the odd part on the EE - but they don't surface often. I'm a long distance from any gunshow but I imagine I'd find some odd parts there.
Numrich occasionally gets some small pieces but rarely. They don't last long. Just another place to look.
Wisener's has SOME parts (I don't think they have hammers) but of course you'll have to use Prophet River or IRUNGUNS - gets costly for a small part if they do have it.
Otherwise, I make my own parts ... just a little milling machine and lathe but more than adequate for these sorts of things.