The Airport card is the most trouble-free solution, but they can be expensive and hard to find. I'm glad ifixit has them.
Alternatives are the PCMCIA card ones, as mister790 suggests; there are also USB/wireless adapters, but I dunno if they'd work with the system the Pismo is running. The USB solution (and some of the PCMCIA cards, I think) means dinking around with drivers and connection apps that are workable but a pain to use.