Originally Posted by Black Sunshine

There is no standard. You need one that is compatible with the shifter that you already have. Or you can find the shifter and derailleur being sold together. You'll need to redo the cable too which means that you will need: cables, cable housing, ferrules, and the little tips to keep them from fraying. Gear cables 4mm, brake cables 5mm. The housing is specific. If you like paying retail Amazon actually carries a lot of parts. If you don't mind buying used Ebay often has whole component sets. If you want it really cheap and like the community thing go to a co-op. If there's one near you. If you want to feel like it is all assembled professionally go to the LBS with a lot of money.