Cheap pc sources wanted

Greetings to the group:

My quest for old 486 or pentium machines to support an EMC
implementation and various other projects continues.

Does anyone have a source for these jewels in the rough? I do have
Boeing surplus available, but I'd think I can beat their $80-100
price. I've also checked out the local Salvation Army type places,
but they won't allow one to look inside the box, which makes them
overpriced at $10 each. :)

I'd prefer to get motherboards with CPUs only, as opposed to complete
boxes, but would be interested in all sources.

Thanks all.