d33ps1x:25 cents at the local gas station. Only need it about once every few months to top up the car tires though.
This made me laugh. When I did my OW certification dives I dove in Puerto Rico on vacation. I showed up at this little out-of-the-way dive shop (that I had never seen before) at 8:00 am and two guys from the shop were out in front with a tank hooked up to the tire of a car. Now, I know they were filling the tire (pressure difference and all), but the thought just made me laugh thinking about filling scuba tanks from the air in car tires. Gives you lots of confidence in the shop doesn't it?