In case anyone else out there is still using these, an online store called 'Cables to Go' currently sells the USB sync/charge cables for the iPAQ 3800/3900 (which also works with most other iPAQ's) for $.40 each. Shipping is extra, but it was $9.00 for the TEN cables I bought. $13.00 TOTAL for ten.
I take them apart and rewire them for RS-232. Tedious, but cheap (hmmm, sounds like ME!)
Cheers,
Uncle Fuzzy