Well, I get to answer my own question. I saw some vague references to Lawn Genie being owned by Toro during my internet search. So, on a hunch, I went to the Home Depot and found a Lawn Genie valve that was identical except in color to the valve I have. And, this one, they had a replacement diaphragm for. The leak is gone, and I've learned that Toro needs to work on their parts availability. Now if only they will learn that.