I have never used an H-Bridge. can you do pwm with it?
and do you need anything other than the h bridge?
Also, be sure to get an h-bridge that can handle the current needs of your peltier - for most (probably all?) peltiers, any simple L293 or L298 based h-bridge driver circuits will -not- be able to handle the current needs (even an L298 wired in parallel for 4 amps of output current).
You'll likely need something with 5 or more amps of current output capability (10 amps to be safe). That's not a small h-bridge, but it won't be horrendously expensive to purchase; you might even be able to build one (simplest would be to use a DPDT relay with contacts rated for 10 amps (easy to get).
You likely won't need any diodes on the output of the relay, since a peltier is not an inductive device (it won't hurt if they are already a part of the device, should you purchase the an appropriate h-bridge controller).