I have this valve on the hose going into the heater core that I use to open/close/regulate flow to the heater in lieu of the Ranco valve. It was around $20 at O'Reilly, and I use a $10 choke cable to work it from in the cab, but if all you want to do is isolate the heater, you don't need the cable. I don't know if there's risk of the system sucking air in through the leak or not.
https://media.fotki.com/2v2aS82LyxDubp.jpgOr you can remove the hoses, loop one (or a short length of another hose) to the hose fittings on the water pump and thermostat housing to bypass the heater.
I assume you don't want to remove the hoses altogether and put plugs in place of the fittings on the water pump and thermostat housing since you intend to fix or replace the core to put it back in service.
