For Happ parts I almost always (actually, can't think of a time when I didn't) go to PlaydiumStore.com. Added benefit for me is that they are local.
You can also email the folks at retroactivearcade.ca, multicade.ca, etc.
Having said that, I don't think you can go wrong with DM and, IIRC, he is reasonable to the point of being generous on shipping.