What optical drive do I order

Pretty much any superdrive with the same housing dimensions will do. I have replaced them with other G5 superdrives heck I have even used one from an iBook G4. Just make sure you get one with your needs for example if you want to be able to burn dual layer disks and the speed of the superdrive, they don't all have the same capabilities. You will have to use the old connector board, optical drive bracket and EMI gaskets as well as the temperature sensor on your new drive. Here is the guide for replacement: iMac Intel 17" Optical Drive Replacement

I hope this helps.