Here's a very nice one (supports both USB2.0 and FireWire!)...
Links not working
Apparently theres a thread called "External Housings" in the "Recording Hardware" section, but I couldn't find it myself (easily )
I have used one manufactured by a company called ViPower which worked fine with an internal CD-RW< but never tried it with a DVD writer. I guess they should work just as well, so long as it's using a USB2.0 or Firewire connection (NOT USb1.0 which is far too slow)