Lynn,
I'm surpised that nobody has answered this, yet. It's a relatively easy one.
Your iMac G3 (officially called the "iMac (Summer 2001)" model) needs an original AirPort Card. It has a part number of M7600LL. There are several variants, such as M7600LL/B, M7600LL/E, etc., but they should all work in your iMac. Make sure that you do not buy an AirPort Extreme card. It only fits newer iMacs with LCDs.
You may also need an iMac AirPort Card Adapter, which connects the AirPort Card to the iMac's innards. It is Apple part number M8753G/A. iMacs that came from the factory without AirPort as a built-to-order option usually didn't have the Adapter already installed.
Now, for the bad news... Original (M7600) AirPort cards were discontinued a long time ago. The supply was scarce to begin with, so the demand is very high. As a result, the price of a used AirPort card now sometimes exceeds the price when it was new! There are vendors selling them online. A Google search for "Apple AirPort Card" will turn up several. Prices will range from $69 all the way up to a mindboggling $250. They are also relatively common on eBay. Usually, the bidding goes into the $65-70 range, but sometimes a desperate bidder will go past $100. The Adapters are usually in the $25-35 range.
Perhaps, your best bet is to try OWC, a well-known and respected Apple reseller. They currently have stock of the latest revision of the card (M7600LL/E) for $69.99. See this link . The cards are new and have a 1-year warranty. OWC does not list the Adapter on their site, but ask them if they have any.
Instructions for installing the card are on Apple's site. See this link .
Good luck.
