One can be obtained from a quest in Broken Hills, given by Marcus if one chooses not to take the money at the end of the quest.
Skeeter in Gecko, Eldridge and Algernon in New Reno can all upgrade a normal hunting rifle. This is also the only weapon that Buster in NCR can upgrade. Strangely, Valerie in Vault City cannot upgrade it for some reason.
While the Pip-Boy description of the weapon refers to the scope as a Loophole x20 scope, Buster claims that he mounts a Triveld x10 scope.