The kit is actually a great buy, you get a MMC->usb adapter that is supported under linux, as well as a 32MB SD card. The Juice Box cartridge that uses the MMC card is nothing but the MMC socket and a pcb, no electronics, which means it can be used in other projects.
The kit retails for $39.99
If you can't find a kit you can hack your own
Now you can also hack the cartridge to boot uclinux
The kit will handle SD cards, but does not implement the extra pins.
- http://www.juicebox.com/checkver100/ - software checks here for updates.