So, the Zipit's add-on storage option was designed by Goldilocks. The micro-SD format, that many cellfones use, is too small. A standard SD card, that cameras and netbooks favor, is too big. The "mini" SD is just right -- and who else uses it, I don't know.
I found a package from Patriot gathering dust at the local electronics superstore. It's a teeny 2g micro-SD card, but it comes with an adapter to make it a mini-SD card plus a second adapter that makes it a regular SD card. Problem solved, for ten bucks.