Good ole Harbor Freight! They didn't have any rotary cutters but they do have good tape measures! For $1.99 I bought a 3/4" one that will certainly do the job for Snap Bags!
I have the "Snap Happy bag" pattern available at a lot of places. Just google that! Has 3 sizes and a few options.
Beware that the pattern says to cut just one length (cut a piece) of the metal tape. Of course you need 2. It also says to have the numbers face the inside of the bag. Most of us have them face the outside so it closes )( this way.