Bike junkyards, I don't know about...but there are some bike coops.

Rusty Spoke | A community bicycle collective

There's also SteveBay (just search for it on Facebook) that is a local group where bikes and parts pop up all the time for relatively cheap.

If you're looking for next to nothing/free pricing, your best bet would probably be garage sales and craigslist (every now and then you find a bike hoarder that's ready to move some stuff).