I did end up finally ordering this one from NewEgg. It seems to be the only one they sell capable of bus power off either USB or Firewire. The USB cable it includes has two plugs on one end (one for extra power), then a single USB mini plug on the other that goes into the device. Firewire is of course just the standard 6 pin (that my Dell brick can't be nice enough to include even with 4 usb ports).

The enclosure isn't all that impressive, but it is functional. I just need to remember to plug in the USB cables to the computer first, as the drive will try to power off one USB plug.