You do have the option of using some broadcom based CardBus cards that Max OS X will consider to be AirPort Extreme cards(like the Linksys WPC54G version 3). You can also get USB ones. Many cheap USB ones use a ralink chipset and those have terrible terrible OS X drivers unfortunately.

When it comes to buying wifi hardware you have to actually get the model number and the version number if possible then look online to see if it will work with OS X. Companies change chipsets all the time so it can be a hassle sometimes.