While you can find USB Bluetooth units for older Windows systems still. Sadly, trying to find a unit for a Mac is almost impossible! The hardware is the same but they mostly don't offer the needed driver software to work with Mac's (older PowerPC or even Intel based systems) Apple was an early adaptor of built-in bluetooth in the laptop systems. So the market just wasn't there to sell into.
If all you want to do is connect a mouse (wirelessly) you could use an older Logitec wireless mouse which has their own USB plug in transceiver. Logitec was one of the first suppliers of wireless mice. I'm sure their popularity helped push forward the Bluetooth standard.
Lastly, given the age of your system (and OS) I don't know what else you could leverage via bluetooth without hitting the PPC Vs Intel problem.