Official Sites
In the "Discography" section of the official site, a few of the albums have Flash movies that contain selected samples - Wonderful Life, SIGNAL, COLORS, and Blizzard Music (ブリザード・ミュージック).
There's an official MySpace page for Yoko Ueno's Tokyo Humming album, with 5 samples from it.
There's now official MySpaces for both Zabadak and Koko Komine, with samples that are updated on a regular basis.

Retailers
Music stores often put up samples with some albums. The only English site with samples for Zabadak I've found is CD Japan. You can find their listings here, and look for any album with an audio icon.

Youtube
Probably the easiest way to sample any music is to hop on over to Youtube. Check out this channel for a compilation of many of the Zabadak videos on the site.

NicoNico Douga
NicoNico Douga is a Japanese video sharing site similar to Youtube. However, you need to be a registered user to view the videos. This blog post details how to register for non-Japanese speakers. After registering, you can view the relevant tags here: Zabadak, Yoko Ueno, Tomohiko Kira.