BottomLine (www.blol.com) has one for $80 or so.
Be warned, though, it's very slow... it's ideal if you have a spare drive lying around that you need to use, otherwise either upgrade your internal HD or get a FireWire drive (if possible).
For comparison's sake, internal first-gen iMac drives work at 10 MB/sec. Second-gen iMac drives are double that. USB drives are about 500k/sec. Real-world performance, of course. Apart from use for backup, they're an external USB drive isn't that practical.