I believe it is a Socket 478, But like the power supply, the motherboard is usally a proprietary fit also. So Good Luck.
I know you probably don't have the money, but if you can't get replacements from the manufacture anymore for a specific model then its usally not worth it to go searching for replacements. Its alot easier to build or buy a new PC, Which could be a good thing in some cases.
You could always scout out ebay or another similar site for spare parts.