Used is your only option at this point, that sockets been dead now for like 5 years.
The processors are also terribly slow even compared to a new $50-$60 Athlon II, I wouldn't advise spending much on it. You can get a decent socket AM3 dual core, AM3 motherboard, and 4Gb of ram for under $200 easily. Possibly even for $150 if you can find some good combo deals.
some 939's dont support any dualcore processors..
I just upgraded a workstation and it required a bios update.. and it still hosed windows.. and I had to reinstall the original processor to install windows... It magically works now .. kinda wierd.
What is the exact motherboard and revision you have and check the cpu compatibilty listing.