Buying AirPort Cards
Apple has apparently stopped selling AirPort Cards (the original 802.11b flavor) directly to consumers, meaning that if you have a Mac model that wasn't revised in 2003 or later, you can't turn to Apple to add Wi-Fi support. (This may be old news -- I'm not sure when they actually stopped the direct sales, but have been reading that AirPort Cards go for $100 or more on eBay right now!)
There are alternatives, depending on which computer you have and which Mac OS version you're running. For instance, Belkin's 802.11b USB adapter has support for Mac OS 9.2, 10.1, 10.2, and 10.3! PowerBooks and Power Macs have PC Card and PCI Card options, even in Mac OS 9; more options are available in 10.2 and 10.3, however.
I have an appendix covering a range of options for alternatives to AirPort and AirPort Extreme for older and newer machines in my ebook Take Control of Your AirPort Network. We just released the 1.1.1 update to the book: it's now $10 instead of $5, but it's 150 pages instead of 89! I added a full configuration guide for AirPort Express among many other additions and improvements.