I don't know of anyone making them, but...
There are already made sawed off shotguns in the NFA registry, and they can be found for sale. There are also the old Ithaca Auto&Burgular guns, which were factory made "sawed off" shotguns, also in the NFA registry.
And I believe that even though the machinegun registry is closed, the "other weapons" registry (where sawed off shotguns are) is still open, and you can get new made sawed off guns added to it.
To be legal, you MUST get BATFE approval BEFORE shortening the shotgun below the legal minimum length (either barrel length or over all total length), otherwise you could be arrested. Once you have filled out the forms, paid the tax and gotten written approval, THEN you can cut down your shotgun into a sawed off and be legal.
Either way, you must get govt approval before you can buy or make a sawed off shotgun. If you do not, you will be in posession of an illegal firearm, and subject to all the penalties of law (up to 10 years in prison if convicted, plus fines).
Your best bet for the current rules governing these kinds of firearms is to cointact the BATFE for the answers to your questions.
Good Luck.