MacResQ has them from time to time (
http://www.macresq.com). Search 'cube' from their homepage.
Also Voelker Research had some used cubes for sale recently in their 'consignment' store (
http://www.voelker.com).
CubeOwner.com is a good source of info and is updated pretty regularly. Also, you can keep an eye on Dealmac.com.
I bought my cube from an ad on
http://www.craigslist.org. If you live in a city that has a craigslist, this might help you.
Another option is to use eBay, but only bid on cubes being offered by local sellers, and only if they agree to let you pick it up and pay for it in person (ask them before you bid). That's how I bought my Pismo Powerbook recently (of course, I live in the SF Bay Area where there are tons of Mac users and used Macs for sale... YMMV).
Good luck.