If your PowerMac has PCI slots in it, then a simple USB card should be sufficient. As for which one, I think that NEC chipsets are supported, but do look through the posts on this site for more info. I use a Sonnet USB-FW card in my PM7200.
If you have one of the first 3 PowerMacs (6100, 7100, 8100), then you are out of luck, until the BusBoy is up and running. The Kensington Mouse-in-a-box (64475) is a good mouse that'll work on both ADB and USB but it doesn't come cheap - a used one on Amazon will set you back $40. Personally, I prefer the Kensington ADB Turbo Mouse. They are roller balls and move nicely.