...make your own, if you happen to have an old iBook/PowerBook/any laptop with an optical drive hanging around unused.
I just got a slim optical drive enclosure off eBay for about $10 and used an optical drive from a laptop I'm not using to create a slim external DVD-/+RW that I can use with my drive-less computers (netbooks and my eee Top). Took about ten minutes to do, costs a hell of a lot less than Apple's overpriced peripheral, and works off one USB port (although a power cable is included if your laptop or desktop doesn't output enough power for the drive; it also uses USB).
Worked damn well for me, at any rate.