Go to Coruscant and at the trooper legacy vendor outside of the Senate and you can find one there.
That's the only one of that specific model that I know about, yeah. Sadly, it's level 14, only really useful for a couple of levels, and requires you have at least one character that's completed their class story act one in order to buy it in the first place. I'm not aware of any customisable variants that can be used long term.
That said, there's a few variations of the same style rifle around, generally with extended barrels, scopes and the like. I'm using this one