You're talking about carriage bolts. They have a square under the base of the bolt head that is supposed to dig into the wood or metal so they don't try to turn when the nut is tightened on the other side. They also have something called an elevator bolt which is the same thing without the rounded head. They are flat and very large head (think fender washer sized).
Both of these should be avaliable in any hardware store. Just ask for them with these names and you should have no problems.
Bumble