If you are still in Duck, try any Grays Store. There is a Grays in Scarborough Faire. You could also try one of the toy stores. I have also seen them in the Farmer's Daughter right across the street. And of course, there are several stores in the "boardwalk" on the Sound Side that have them.
If you are home, you can try the Gifts & Souvenirs page in Outer Banks Revealed. Go to "Outer Banks Gear & Wear" and click on OBX Bumperstickers". I ton will show up, but go up to the search and enter "State Route 12" and you will see your selection.