Discussion on a fairly high powered inexpensive H-bridge.
http://forum.arduino.cc/index.php?topic=148482.0
http://www.ebay.com/itm/Double-BTS7960B-43A-Motor-Driver-High-power-module-smart-car-driver-For-Arduino-/221235721147?pt=LH_DefaultDomain_0&hash=item3382ad33bb