I have a pair of -79's and a pair of 2079's, and I have never had a leaking problem in any of them. Do you mean to take the gas bottle off between sessions? Is that why you want an on-off valve?
There are lots of adapters for PB-AS, AS-PB, and you can get them with on-off valves, burst discs, etc... Nice thing about an adapter for AirSource to Painball (AS-PB) with on-off is that you can refill your AS carts. I don't use AirSource, don't like 'em. If you're using a standard paintball tank with a pin valve, you don't really need on-off, because you can just twist the tank off when you're done - you'll lose gas, but not the whole tank, like with AirSource.