Here is
one supplier of DIY kits, Google "pole barns" in your area.
I would suggest making the structure with at least 14' interior height for possible future needs. My neighbor has a beautiful 20x40 garage with a 12' tall door where he used to park his trailer. He sold the TT and talked about restoring an Airstream when he retired. Now that he's actually retired, he shifted gears and bought a Class C and it won't fit in the garage.
The standard for interstate overpasses is 13' 6" so if build it with 14' of headroom you'll be "future-proof" if you decide down the line to get a 5er or motorhome.
