You have two basic options. First, you could buy a PCI wireless card-there are a lot of them out there. You would have to make sure that the card came with the correct drivers for your version of MacOS (something that is problematic for a lot of Mac users), and of course make sure that the card would work with (that is not cause a conflict with) all your other PCI cards.
Second, you could buy a basic ethernet card for your Mac (if it doesn't have a working ethernet port already), then buy an "ethernet to wireless bridge" like the Linksys WET11, the SMC 2670W, or the D-Link DWL-810+. These products are configured through your browser, so there's no worry about drivers, etc., and they just sit there and work; there's no real need for any management or upkeep.
You can probably tell that I've been frustrated by trying to find (affordable) PCI wireless cards for Macs. Yep. But with these new bridge products from several vendors, I see a light on the horizon, (music starts here) where any computer user (music gets louder and more dramatic) regardless of his or her platform (louder, more dramatic, more inspiring!) can wirelessly network his or her chosen computer! (creshendo!) Thank you. (applause!)