There might be a limitation as to which CD-ROM drives you can use to boot. The drive may have to support a somewhat rare(?) 512 bytes per sector mode. While drives that don't support that mode will probably still work once in AIX, they cannot be used to boot from. Most of the IBM one's will boot, as well as some Plextor and Toshibas. If you get one to work, please send me the model/part-nums. Tested working drives include: Toshiba XM-6401TA, Panasonic KXL-RW10AN-S burner, Panasonic KXL-810MN. Probably will work drives include: Plextor PX-12TSi, PX-20TSi, Toshiba XM-5701B, Toshiba XM-6201B, Sony CDU-561SC, Toshiba XM-3401BMA.
You can turn most generic internal drives into an external by buying an external SCSI enclosure that supports the size of drive you are using. Any old SCSI-1/2 enclose should suffice and can probably be found quite cheap on ebay (make sure you get one with a working power supply). Cabling might be tricky and/or expensive so make sure you get the correct one.
Thanks to Juergen E. Fischer and Stefan Tibus for the tips.
2013-09-22 Another good source for parts is www.cygnussupply.com but you must know the part numbers to search for.
Thanks to Antoni Sawicki for the tip.
Please e-mail aixtp20@tecnopolis.ca if you have any comments!