Hi,
your G3 iMac will accept only the original Apple Airport card, NOT the Airport Extreme card.
These cards are no longer made by Apple and were never made by third party vendors. Unfortunately the demand for them is fairly brisk so the prices are mostly higher than they were when new.
You will also need a adapter for the iMac to hold the card in place.

I just did a Google for "original airport card" and the found quite a number of hits.
http://store.airportcardsolutions.com/apaica1.html is just one and they sell the adapter.

This is not a recommendation, merely a starting point for you, you may be able to get a better price by searching.

On the other hand, you can always wire the iMac directly into the router with ethernet cable.

P