buy a moto razor cell phone charger off ebay for $1.99 and call it done.
the razor chargers are dirty cheap 'cause the phone is so popular. the charger already contains an efficient switch mode buck converter, and since it's for the razor, it already has the mini-usb connector on it.
if mini usb doesn't work for you, you can lob the end off and solder on whatever end does work.
or yea, you can hack together something using a cold-war era linear regulator like the lm317 or easier still the lm7805. it'll waste more power than it provides.