With 5-20A per circuit, your best bet is going down to the auto parts store and getting a fuse block. They are available with various numbers of fuses, usually using the standard automotive blade fuses, which I believe to be compatible with the FIRST small breakers. If you were looking for larger circuits (e.g. 40A), I would look for a MAXI fuse block sold on-line for high-end stereo and similar systems.
Another good place to look is in a marine shop or in the marine equipment section of your local hardware store. Marine departments and auto shops are great places to locally source FRC and FRC-similar electrical items.
Edit:
Motor controllers: I'm using the SPARK controllers controlled by an arduino (using the servo class) for a small project this summer. If you're looking at smaller motors, you may be able to use a
motor controller shield for arduino. (I believe there are other shields out there that have slightly higher current capability, but I don't recall seeing any in the 20A range.)
Edit2:
Controller: I understand that the Pi does not have any hardware PWM circuitry, making it a poor choice to operate a motor controller. I'm planning both a pi for vision processing and an arduino (leonardo compatible) for the hardware control side of my summer ball launcher.