Another idea, especially for those with old magazines: what was the first USB possibility for the Mac? I had thought that it was the iMac. But now that I think of it, there may have been a third party card *with their own drivers* that came out in 1997 or 1998. I was a diehard SCSI/ADB man at the time and I had learnt about FireWire in the reviews for Mac OS 7.6.

If there was a very early USB card with mac drivers, then eBay et al is the place to scour. Also, it might be worth looking out for the PowerMacs of the day (e.g. PM 7300 | PM 9600 | Beige G3 etc) that may have been equiped with them when they were bought. You would need to find a buyer that bought the machine to be sure.