Here are some alternative arduino boards that can be used for the wonderful pairing of arduino + nrf24l01.

For sensor (arduino+nrf24l01 header) - Sensor Node by Elecrow,
http://www.elecrow.com/wiki/index.php?title=Sensor_Node_V1.2_(ATmega_328)

For gateway (arduino+ethernet+nrf24l01 header) - IBOARD by Iteadstudio,
http://imall.iteadstudio.com/development-platform/arduino/arduino-compatible-mainboard/im120410001.html

Please add any other hardware options you find and that make this project even simpler to implement.

Battery Options (for sensors, regularly sleeping)
-1 alkaline battery + Step-up Power Converter to 3.3V
-2 alkaline batteries in series or button cell lithium battery like CR2032 (3V)
-LiPo battery (3.7V)
-3 NiMh batteries in series (3.6V)

Non-Battery Options (for gateways and repeaters, always on)
-USB Phone charger + Step-down Power Converter to 3.3V