Can I attach this flashight to my USP without an adaptor? If not, where can you pick up the proper adapter?

You need to get an adapter for the M3/M6 on the USP.  If you are only getting a light you can get the UTL.  Both are made by ITI and I believe its the same lamp.  If you are looking into getting the M6 on the other hand the GG&G adapter is great.  I have it for my tactical and so far i think it is awesome.  Sidearmor also sells an adapter but i thought it had too many parts for my taste.  There were several dicussions and pictures about this at hkpro.com .
You can get the GG&G adapter directly from GG&G or Botach.  I beleive I saw some one selling them for like 53 bucks either here or at the hkpro EE forum.