kleptophobiac's suggestion is the most sensible, but I think most of them will occupy two slots... Another option would be to get one of IBM's Travelstar external HDD-to-PCMCIA or similar, you can usually get a small one (<5GB) for well under $50.
As everyone else has pointed out, most systems don't support booting from USB but if yours does, you should have a look at
Damn Small Linux. It will need to be hacked to include Kismet and the wireless goodies, but it's under 50MB so there's plenty of room