Power Supply Options:
You will need a power supply rated at 5VDC and at least 700mA (0.7A) and with a mini USB type plug.
 A 1000mA (1.0A) should work for most people.
My LG cell phone charger works well.

 I also have a 5VDC 10A supply that I got from a surplus store many years ago.

Beware that many chargers were designed to charge batteries and may not work with the RPi as they are designed to revert to trickle charge when the batteries are fully charged. Since the RPi is not a battery load the charger may become confused. 

I bought 2 identical Ebay chargers for a few $.  One worked with my RPi and one did not but they both worked with my cell phone. They had slightly different output voltages. I believe that many 'cheap' deals on Ebay are factory 2nds. I was able to pop the cases off of the Ebay supplies and unsolder the power cord and use the cords with better power supplies. Probably couldn't have bought the power cords alone for any less money...