I made a couple of leashes for our dogs' trainers for Christmas. They loved them, but one suggested this type of snap. She's had problems with dogs slipping out of the others (she tends to take in the rowdy dogs - a test of skill and patience, but also a reflection on her dedication to dogs that many, if not most, would dismiss). It sounded as though she does have leashes with these trigger snaps, so they must be commercially available, though I don't recall seeing them anywhere. Certainly not at any of the big-box pet stores.
This probably doesn't answer anyone's questions, but the suggestion was enough to make me go out and buy trigger snaps for the rest of the leashes I'm working on.