My dog bent off some of the vinyl/metal of the back door to to the backyard, Where do I buy metal doors.?

Home Depot carries them,You may have to get a pre-hung rather than a slab. You will be better off going to a door store as it is not as simple as popping a door off and popping a new one on. You have to mortise for the hinges and bore out the door for the lock set and deadbolt. You may find an exact fit,but the chances are slim. And no It wont be really cheap.