It all comes down to the current requirements. And the cash available, I suppose.
Sparkfun has a motoduino shield that handles two 2A motors for about $25.
You can build one that handles 2 channels of 2.5A each for about $12 - I have the schematic and a PC Board design if you want.
And there are more options, depending on whether you have an electronics junk box or not.