You can get a standard storm door in a 36x85 size, and the miniumum/maximum size doorway height that would accomodate that size door would be 84" to 85". Why would you need a custom door?

Oh, also, if this helps......Our last house had a doorway height of 78". My husband purchased a standard door and trimmed the bottom of it, as well as the side pieces, then put the bottom molding back on....it was impossible to tell that it had been trimmed down and looked great. When we purchased the door we just had to make sure that the hinges weren't the piano style that go all the way down to the bottom. Instead, we picked a style that had the 3 separate hinges, and the bottom hinge was high enough up so that it wouldn't be affected by us cutting the door size down. It was so much cheaper than purchasing a custom-sized door.