Do you know what socket you need? AMD or Intel? I assume you have a complete setup sans mobo.
If you're looking for 939, check out eBay. It's flooded with AGP 939 boards from people who are upgrading to either PCIe, AM2, or Core2's. Like uber said, if you can find one of those Asrock AGP+PCIe boards, that would be a good path to follow. I've been trying to snag one myself on eBay, they usually go for about $50.
Ascendtech has the ecs kv2 lite for $50. I use it in my backup system. The 939 cpus that run with it can be had for around $37 at starmicro or newegg open box. Only problem I had with it was the onboard sound. You have to load the latest driver off ecs website. The driver that came with the board crackles. Latest version fixes it.