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.