For modders in the US, give your local Goodwill a call. Some of them have started a new store called Computer Works. They're recycling computers and selling them as whole systems or parts. I just got back from the one in Houston where I picked up a Linksys WRT54G for $20. They've got MB, CPUs, PSUs, cables...pretty much everything. As you might imagine, much of its older versions but for modding, the potential is huge. They probably had 50 or 60 cases on hand; some of which were server and other non-standard form factors.
The guy who runs the place says that they have stores in Texas in Austin, Houston, and Waco but there is similar stores in Massachusetts and California that he knew about.