Standard buck switching power supply from the likes of Linear Technology, TI, Maxim, National Semiconductor, etc.
You might come up a little shy on the output side, though -- you're asking for 90% efficiency in the conversion, and this is high enough that -- while quite doable -- you will have to pay careful attention to where your losses are. Using something like the National Semi. 'simple switchers' probably won't cut it. (Although I expect they would readily get you at least 80% conversion efficiency.)
---Joel Kolstad