I have an iMac (21.5-inch and 27-inch, Late 2009). I need a replacement superdrive

Unless you feel equipped to disassemble the computer in order to replace the drives I would suggest getting an external optical drive instead. Otherwise, I would take it to the Apple Store for the repairs.

If you wish to do the job yourself, then search Google for links and videos about disassembly. You can get toolkits and optical drive replacements at OWC.