I prefer the SWC DX2 from http://www.front.com.tw/english/E-home.htm . If you get this ask them for a 64Mb unit (used to be $20 more than the 32Mb unit).
If all you need is a 32Mb unit and you don't mind loading all your games from floppies (because that's the only way to do it on the Super UFO) then get the Super UFO Pro 8 from Lik-Sang. It is cheaper and is just fine if you can live with the above limitations.
With both of these units you'll want to pick up an external power supply to use with it. You can buy one at Radio Shack.