I got an original heater control valve from a semi-local shop here in Denver that needed to be rebuily for $20. The valve mounts on the rectangular hole just above and to the right of the heater motor, in the engine area, as seen when looking at the truck from the front. The control rod that bolts to the underside of the dash controls the valve, he got me for an extra 5 bucks for that. If you get ahold of one of the valves, you can order the rubber diaphram that is needed to rebuild it from Chevy Duty($15) or most of the suppliers, but it comes in a NAPA bag, and you can get it from NAPA for $8 or so. I don't have the part # handy, let me know if you need it.
If you aren't concerned with originality, just get a ball valve with some connectors and cut the hose that leaves the motor, insert the valve, and you've got a shut off.
Good luck!
