My mo lives in St. Ignace, just across the bridge, and she has a hard time getting any craft supplies. She usually ends up getting what supplies she can find at the Wal-Mart in Cheboygan, so I don't think you will have much luck. All I saw in Mackinaw City when I was up there last week was tourist shops. Now, if you want to know where the get some fudge...
Enjoy your vacation!