It is unlikely, as far as I know, that you'll find any usb wireless set able to actually work on Mac OS 9. What I'd do is mosey on over to your router and plug the iMac into the Internet with an Ethernet cable, or find an Airport card and install it into the iMac. To my knowledge there's no way to get Wireless-G on that particular iMac, especially since it's USB ports are only the 1.1 variant anyway, so you wouldn't get G speeds out of a Wifi-G adapter.