Ebay is a good place to find things, also, geeks.com has a lot of used and refurbished parts, they are basically a discount outlet for geeks. I picked up an Acorp 6A815EPD mobo form them for 25 bucks and it supports dual Socket 370 P3s on an intel chipset that wasn't supposed to allow for duals. An old but good roundup of parts is here.
http://www.anandtech.com/mb/showdoc.aspx?i=1522
I'm running 2 PIII 800EB on the Acorp board, back when I built it, the CPUs were 80 bucks a piece, damn Intels stay pricey forever, but now they are on the cheap though.
If you want to go dinosaur, look for an Abit BP6 and run some dual celerons. Even more dinosaur would be to get a dual/quad pentium pro board and throw some 200mhz ppros in there, which is cool if you can find the 1MB cache chips. But make no mistake about it, a comp like this will be slowwwwww.
If you have the $$ and/or patience, I would look for some old AMD dual MP boards, like a Tyan s2460 or something. You can run normal socket A XP, duron or T-bird CPUs in a dual config, with some slight modding (closing the L5 bridge if it is cut, some old ones weren't). The catch is those AMD dually boards usually need Registered RAM, which is more expensive, and a good power supply that devlivers enough current on the right rails. But, they are way better than dual P3s.
While P2 and P3 xeons may be cheap CPUS, the problem is finding a cheap board that supports them, and then finding VRM modules to throw into the mix. You may be able to pull off a cheap dual xeon rig, but dual P3s are gonna get ya better price/performance, and AMD will top that. The only reason to get P2 or P3 Xeons is for Quad or Octal systems because P3 Pentiums are dual-capable, but not quad-capable.
But honestly, I would keep my eyes on geeks.com and ebay if I were you. A good deal will come along eventually.
