Ok, I know this may not be applicable to everyone, but there is a place here in Phoenix that I absolutely LOVE for motorcycle parts called Bob's Used Motorcycle Parts. It is basically a salvage yard - but not your typical yard.
The owner runs it along with some of his sons and some other guys, and they really know their stuff. They pull most of the frequently demanded parts, and you can just go in, tell them what you need, and most of the time they will come back with something ready to go.
If they don't have it there, ready to go, they invite you to walk the yard yourself and see if you can find what you need. You can't pull stuff yourself - you just tell them where you saw what you need, and they get it for you. And did I mention they have THOUSANDS of bikes? They also carry a nice selection of new parts - things that people typically want/need, like mirrors, cruising pegs, lights of all kinds, etc.
Today, I fell in love all over again when I walked in, needing a carb for my bike. Bob said, "hey, didn't someone bring one of those in recently?" after the guy that was helping me didn't find anything. They both proceeded to dig around, and about 5 minutes later - VOILA! A complete, working carb assembly for a Vulcan 750. And the even better part - ready for this? - $100 out the door! Yeah, I couldn't believe it either.
So, if you are ever in Phoenix, AZ, stop by and check it out. I am listing the phone and address below, however, forewarning : I have never actually gotten through on the phone - always get some message about them being too busy. So unfortunately, it may not be a good option for out of staters. That's not to say you can't try though. I know it would be worth it.
Bob's Used Motorcycle Parts
1325 East Elwood Street
Phoenix, AZ 85040