This might be a tough one. Your best bet (other than eBay, on which you might lose in terms of shipping) would probably be to visit surplus retailers like Geeks.com.
I've been on the lookout for affordable socket 939 (AMD) motherboards, and they're either not to be found or cost more then I'd like. $30 seems a reasonable amount to pay for obsolescent technology. Above $50, you start thinking you could buy a modern motherboard with integrated graphics (see Gigabyte's 945GCM board), pop in a Pentium E, 2 GB of cheap DDR2... Well, it's just very hard to justify spending $50+ on older tech, given what you can buy for < $150 these days.