To purchase separate from the gun?
Buy the matching gun with the interchangeable mandrels, etc.
Then you can have TWO nut-setters in different sizes, ready to go!
If yer really bent on doing it cheap, it's not hard to work out the needed dimensions, you need to match to the threaded stud that has the knurled knob on the back end of it (used to change mandrel sizes, and have a threaded portion at the nominal size of the insert thread.
The collar that fits in the nose of the tool, need not be anything special for accuracy either, it just provides a shoulder against which the nut insert is pulled against. Other than the issue of the collar dropping free when you forget to pay attention to it, you could make that as a simple hat section bushing and skip the threads.
Personally, being the basic model of cheap SOB that I am, I would build the puller mandrels as I actually needed a particular size, probably by dint of simply modifying a bolt of the correct threads size, and stock up on inserts, as needed, or when they came on sale...
Have used a Rivet puller for installing Riv-Nuts. Sawed the head off a long shank bolt, screwed the insert on the threaded end, and inserted the shank into the puller. But we had air over hydraulic pullers that would deal with 1/4 inch and larger rivets.... $$$$