Try Michael's or AC Moore; I got a thin steel template at my local Michael's, for something like $1.99. You might find an appropriate template at HobbyLobby, too. I don't recall having seen any at my local Hobbytown.
Beyond that, try artist supply or drafting supply stores. We have a local store from the *** Blick chain, as well as a couple of local independent stores that carry templates and flexible rules that will work.
Hope that helps!
Brad