From OWC , USB wireless adapter. I use one with my 1999 iBook.
With full OS X support for Plug and Play operation straight from the box, the MAXPower 802.11g/b Wireless USB Stick Adapter offers fast and stable wireless networking at home, at the office, and at mobile hotspots wherever you travel. It's truly an ideal solution to easily add wireless networking capabilities to or upgrade existing performance from PowerMac® G4, iMac® G4, PowerBook® G3/G4, and/or iBook® G3 models that didn't include built in wireless or has a non-working AirPort® card.
My iBook is a g3 with USB 1 and works reliabily. These adapters also come with their own software.