I've not used it, but this one http://www.ebay.co.uk/itm/240W-High-power-H-bridge-PWM-Motor-Driver-Sodule-smart-Car-Driver-For-Arduino-/170915420830?pt=LH_DefaultDomain_0&hash=item27cb5a5a9e is economical and appears to be based on this chip http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00268302.pdf.