I don't think anyone carries 939's commercially anymore; if that's the route you want to take, your probably going to have to take your chances on ebay. If you don't mind upgrading, you could probably get away with just buying a new mobo, cpu, and ram (ddr doesn't work on ddr2 boards).
This mobo/cpu combo is $120 (athlon II X2 240-2.8GHz + GIGABYTE GA-MA785GM-US2H)http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.254104
This ram is $36 (2 GB-minimum)http://www.newegg.com/Product/Product.aspx?Item=N82E16820231087
This ram is $65 (4 GB-Optimal)http://www.newegg.com/Product/Product.aspx?Item=N82E16820231166