www.icni.com
they have the slot loading drive you need for about $100-120 I believe.
Very easy to put in, you simply open the bottom of the iMac (6 screws)
remove the metal cage (4 screws)
remove the drive cable
remove the four screws that hold the drive carrier on
remove the four screws that hold the drive in place.
replace the drive with the cdrw, and reassemble.