lots of affordable h-bridges on ebay like below.
http://www.ebay.com/itm/Arduino-240W-High-power-H-Bridge-Motor-Driver-Module-Smart-Car-Driver-/330817755465?pt=LH_DefaultDomain_0&hash=item4d06465949
http://www.ebay.com/itm/43A-DC-Motor-Driver-Drive-Module-H-Bridge-PWM-Contro-For-Robot-Smart-Car-Arduino-/300818612987?pt=LH_DefaultDomain_0&hash=item460a2fc2fb
http://www.ebay.com/itm/30A-H-bridge-Coreless-Motor-Driver-Forward-Reversion-Brake-For-Smart-Car-Arduino-/180983568079?pt=LH_DefaultDomain_0&hash=item2a237616cf
http://www.ebay.com/itm/Dual-H-Bridge-Optoisolate-ATMEL-ATTINY-2313-based-Motor-driver-Arduino-PIC-/271028808736?pt=LH_DefaultDomain_0&hash=item3f1a936c20
http://www.ebay.com/itm/50A-Single-H-bridge-Motor-Driver-Module-PID-for-Arduino-Intelligent-Car-Robotic-/230888425765?pt=LH_DefaultDomain_0&hash=item35c205c925