Check out the Raspberry Pi section http://www.adafruit.com/category/105
You'll find they sell a power supply: http://www.adafruit.com/products/501
and the Micro USB cable that's needed: http://www.adafruit.com/products/592
I did see the USB power supply just popped up as a new product so hopefully it works out as well as this guy says: http://www.youtube.com/watch?v=XX3kiRUf7mg
(long video, basically this guy says apple iPad/iPod/iPhone power supply was low voltage and the Adafruit one worked great).
You can also feed power back in through the header, but you won't have any reverse protection or over current (fuse) protection (http://www.raspberrypi.org/phpBB3/viewt ... 44&t=10500
). You could try out http://www.adafruit.com/products/276
if you want to go this route.
If you're looking for something all buttoned together you'll need to try out some of the new phone or tablet chargers with the micro USB connector.
Personally I've been powering my Raspberry Pi with an iPhone power supply. I tried from a powered USB hub but either the power supply was poor or the hub has some protection circuit interfering since I noticed the LEDs pulsing a bit.