That would depend on exactly what is wrong with it. Sounds to me like you've got a clutch issue.
Do you hear anything when it jerks? Grinding/clanking sounds?
Note: A repair shop should be able to tell you - for no charge - what's wrong with your transmission and what part(s) it needs.
http://car-part.com I think would be your best way to find cheap parts. (Also, you might double check to see if the shop you go to installs salvage yard parts in the event that you do purchase something online. Some shops policies say that they're only allowed to install new parts from their own distributor to avoid warranty issues.)
-Zach-