the CPU has a number of pins in a pattern thus giving it a name (socket) if you have the owners manual or do your homework you can find out what the socket it called. some names are (socket AM2) (socket A) (socket 754) once you can find what socket you have you can match it with a mother board that excepts it. maybe one that holds more ram, different ram, dual video cards, on board video / audio and so on. i have a few links below that i use when i need parts. i look for the best deal. maybe them links will help when you need it.
Tiger direct.
Computer geeks
New egg.