Big relay like this one3 pole double throwhttp://www.mpja.com/prodinfo.asp?number=18255+RLhttp://www.mpja.com/download/18255rl.pdf
Good call Lefty.Maybe 4 of these then, wired up like the transistors in an H-bridge?(which I am sure I created a drawing of somewhere)http://www.mpja.com/prodinfo.asp?number=17155+RL
Using four SP relays is not a safe thing because you can't depend on all of them to have exactly the same switching times. If one relay is "slow" you could end up shorting the power supply. Much safer to use a DPDT relay to control direction and a SPST relay (or transistor) to control ON/OFF (or speed if using a transistor and PWM).
...Actually, it would be best to use two SPDT relays, with the lever of each connected to the motor, and the NO contacts connected to +V and the NC contacts connected to GND. This way, you could switch either one to control direction, and both off (or on) for brake.