arduino uno - SIM900a Mini Modem, IMEI 0, help with TX RX pins - Arduino Stack Exchange
ITEAD SIM900/SIM900A GPRS/GSM Module - RobotShop
SIM900 GPRS/GSM Shield Development Board Quad-Band Module For Arduino Compatible : Amazon.in: Computers & Accessories
Amazon.com: Module,Development Board Module Kit,5~26V SIM Development Module,Fit for SIM900 850/900/1800/1900 MHz GPRS/GSM,Quad-Band 850/900/ 1800/1900 MHz - Would Work on GSM Networks : Electronics