The strobist kit is quite good to start, $10 at B&H and several other sites. Pick up a gray card while you're at it, as it'll help you develop a more theoretical way of determining ambient color temperature/shift than guessing/experimenting.


I usually just buy the full sheets from B&H, once I know what colors I use a lot. You'll want CTO in 1/4, 1/2, 3/4, and full, and a dose of 1/4 and 1/2 plusgreen. I don't use the blue gels much, if ever, but they can be useful on cloudy days.