For airsoft use (I also use one on a real Colt 933) I would recommend this from Crosman, of all places:
http://www.crosman.com/airguns/accessories/gear/NS0423
It is an aluminum sealed housing, takes two CR-123 batteries, is plenty bright, and comes with a rail mount and pigtail with pressure switch. The normal end cap has a pressure switch on the end of it.
It costs $15.99 USD with all of the accessories included. I saw this at a gun show, and my friend bought it. I thought it would not be good for the price, and I was definitely proven wrong. I used it at Pine Plains, OP Red Reign, and several other big ops with night games, and so far it still works great.