Don't bother trying to find one.
You will waste an entire weekend looking for a solution that works. You will learn about chip sets and obscure engineering revisions that shouldn't be clouding your brain. You will buy a card, try a card, return a card, buy another card, try another card, return another card. You will get desperate and download lots of third party software. Then you will try to get them off your hard drive to see if native drivers work. You will read countless postings to forums describing success using the same card you just bought but can't get to work with your Mac.
You will spend more than two hundred dollars.
Finally, you'll go to Best Buy and purchase a Buffalo High Power Turbo G Wireless Ethernet Converter (model #WLI-TX4-G54HP). You will plug the unit into your computer's Ethernet port, turn on the computer, see that you have a connection then start writing about what a horrible, fucking nightmare it is to try to get wireless connectivity for your G4 tower.
It was so easy I thought I would puke.
It was too easy. I've grown accustomed to the pain so I'm going to try to update my OS now.