Only things you would find Laser-wise to fit directly to your USP Proprietary rail w/o adapter will be from Laser Devices Inc.
*Blast 1
*Blast 2
*BA-6
Blast 1 & 2, I believe, are only formatted in Laser/Light setup or Light only. No Laser only option with those from what I know.
BA-6, I believe is discontinued, but you can sometimes find them on here or Ebay. THis is (from my understanding) the only Laser that mounts directly to USP rails. You can get this as Laser only, or you can get it with a rail on the bottom to mount a ring and flashlight. Pretty sweet looking IMO
These arent exactly cheap but are the only ones I know of unless you start looking at cheap, flimsy, chinese airsoft lasers.