There are companies that distribute steel doors for the constuction industry. Here in the St. Louis area, there are about 5 of them. Although some of the oddball stuff isn't something they would have in inventory, it is something they can order in.
You may want to Google around a bit to find the manufacturers, and then use their dealer locator to see who is near you. There are several types of steel doors made for prison use that aren't steel bars. They look like regular doors, but are made very tough.