Where can I buy a fire alarm drafting stencil?

I would call the item a "drafting template" or "drawing template." Try searching with those terms. If you can't find the one you want, a general electrical template might have some of the symbols you need. If the one that you like is no longer made, you might try eBay. Pretty much everything turns up there sooner or later. You might call Wheelock. The item was probably custom made for them for internal use and/or as a marketing promotional item. They may not offer them for sale, but the right person might give you one.