Probably the easiest way to obtain the weapon schematic is at the F.Scott Key Trail & Campground. The schematics are located in a trailer usually guarded by a Deathclaw. This makes it the perfect area to get this weapon early on. Not only do you get the schematics, but you get the Deathclaw Hand which is hard to get early.
Another reliable way is to do the Council Seat quest in favor of Bannon in Rivet City.