Assuming you're using a plastic hose for the primer line, you might find a nylon inline valve in the aviation aisle of your local True Value (or other) hardware store. It's what I used for the same purpose on an ultralight. G'luck!
Brent, thanks for the reply. I don't believe there is an aviation aisle in any of our hardware stores but I will look. You would think that a valve like this would be readily available but I have not been able to find one
The primer pump is one that typically comes from Aircraft Spruce or one of the other aviation product companies.
What is the 'locked position"? Mounted vertically, mine is up to pump and down at end of pump stroke. The reason that I want to put a shutoff valve in it is that in the event of the internal check valve failing, the carburetors can suck fuel through it and choke resulting in engine failure. I am under the impression the the primer pump is a low quality item and can fail. I just wanted to be proactive and prevent a possible problem.