I would just find an ADB mouse. ADB to USB adapters were made for a short time following the introduction of iMac for people who didn't like the USB keyboard and mouse Apple provided, but they were not that many made and are very difficult to find. I would bet if you put a want-ad up on your local craigslist, somebody would probably give you one. At this point, most are being trashed anyways. eBay would be the next bet.