[Al Skierkiewicz]

Double Pole, Double Throw relays are available from a variety of sources, many of them cheap. The tough part is you need a contact rating that is higher than the current you expect to deliver to the motor. Be aware that the contacts will arc when changing directions unless all power is removed and the motors are stopped. It is just the nature of the beast. Remember that two single pole relays wired in parallel will be a double pole relay. Digikey PB527-ND relays can be used for this, they have a 12 volt operation on the coil and 40 amp contact rating and at $3.91 each appear to be a pretty good bet for most applications using robot motors.