I am screwing with one of these right now myself.
I contacted Sharp as I had the same problem and they sent me to sdgsystems.
I was emailed back in just hours- great support, esepcially for old, discontinued stuff THAT THEY DIDN"T MAKE!
Upshot is, they gave me a link. http://familiar.sdgsystems.com/pub/zaurus/
I have flashed with that just fine, and all worked well.