Adafruit industries has a nice motor shield that supports up to 4 bi-directional dc motors, but I think its current rating is too low for you...
"4 H-Bridges: L293D chipset provides 0.6A per bridge (1.2A peak) with thermal shutdown protection, internal kickback protection diodes. Can run motors on 4.5VDC to 25VDC."
Anyway here it is:
http://www.adafruit.com/products/81
my 2 cents.