I think it's coz the zip100 never really took off. CD-RW came along too soon, and at 700 Odd Mb per disk, with cheaper media, zip just got left behind. pity really, as better that the old 1.44Mb floppy that still survives, against all odds really.
There are plenty on eBay, but of course thats a case of 'buyer beware'. but at 99p each....
just click here and search for 'zip disk 100' for shed loads.
don't know what they end up selling for in the end, as i took my zip drive out months ago, when i got a CD-RW drive.