You can still find old ZIP 100 drives on eBay, for not much money. Be sure to get one that has a USB connection, and NOT an "SCSI" connection!
The only possible problem might be having the drive mount on the desktop with a new version of the MacOS for "lack of a driver". But I believe this can be "worked around" by putting the ZIP disk into the drive, then restarting the Mac (ZIP disks have an "on-disk" driver that should load during boot time, allowing the disk to mount on the desktop).
As kohlson mentions above, another way might be to take the old ZIP disk to someplace that might still maintain a working ZIP drive.
Perhaps an "independent computer shop" that still services Macs might have an old Mac with a ZIP drive in it, and "copy over" the files to a USB drive (for a small fee).