I bought one of these for Arduino use with 12VDC supply power:
Car Adapter Power Supply--7.5VDC 1A ("supposedly")
It will control 12 to 24 VDC down to 7.5VDC, which the Arduino voltage regulator can then easily handle.
And $0.50 is hard to beat! (I would have bought more, but they weren't that cheap when I got one)