The original Apple Airport card was supported on all iMac G3s, though the very first revision of iMac G3's will need a riser-card installed to allow an Airport card to be plugged in. Both the riser card and the original Airport card are no longer in production so getting your hands on either will be hard. Since Apple stopped selling the orig Airport cards over a year ago, they've actually gone up in price in many places!
Most USB based wireless solutions will work as well. I think you'll find D-Link makes a suitable USB based wireless connection. Unfortunately, you will probably not have much luck with either PCMCIA or PCI based wireless cards as neither of these slots is common on a G3 iMac.