The Mod NX rim cannot be bought separately.
To buy the backplate and/or buttons separately, see below:
Backplate: If you are looking to buy the backplate only, navigate to the Mod NX case design you would like, and then choose the "Mod NX Backplate only" option. See the image below for reference.
Buttons:
On the main page, the buttons can be found by selecting Products>Phone Case>Button