Join Enclave. Or you can steal MFC from Super Mutants encounters aswell.
Stats for thief: +200% steal, the more the better, agility and luck affect (probably)
Laser rifles are easy to get from 2 types of encounters:
1) Unity Patrol vs Rogues near BH - best to do in sneak mode, just wait till they finish each others or you can go with some BG avenger and finish 'em up by urself.
2) Super Mutants Remants vs Centaures/Floaters/Deathclaves near Mariposa - same tactic as above.