Original Airport cards are stupid expensive. If you can spare a PCMCIA slot, there are 802.11b cards that will either show up as Airport cards, or have their own compatible drivers. Right now my mind is drawing a blank on the brand name of the former.

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The original Orinoco/Wavelan cards from Lucent use the same internals as the Airport cards (also made by Lucent). Orinoco cards were also rebranded by folks like Dell (as the TrueMobile something-or-other), and these are also electrically compatible with the Airport card. The problem is that these non-Airport cards have built-in antennas; the card extension to accommodate those makes the card too long to fit in the Airport slot. There are instructions on the web for how to shoehorn these nonetheless, with varying degrees of success. If you do succeed in fitting the cards in there, you will find that "it just works." The card will be seen by the OS as an Airport card, so no third-party drivers are needed.