Plasma rifles are some of the sexiest guns in Fallout New Vegas, and the rare / unique weapon version of it, the Q-35 Matter Modulator, is even sexier. Watch this video to learn how to find it in REPCONN Headquarters.
From the Creator:
Located in REPCONN Headquarters, in the shipping section. You have to pick a Hard door on the second floor to get to it, or pick a Very Hard door on the first floor. Found in perfect condition.