Gotta love google:
H-Bridge Motor control
That will tell you how to make your own speed controller. Note that using that, you can't use the standard program. You'll need to generate a 1% to 99% duty-cycle PWM signal plus a direction signal, probably on the digital/analog ports.
Alternatively, try hobby stores, like:
Tower Hobbies
Horizon Hobbies
Note that you're almost certainly looking for brushed motor controllers, not brushless motor controllers. They're vastly different and incompatible. You'll also likely want one with reverse on it.