Not sure about where to find a "thank you" cookie cutter, but have you tried looking for alphbet letters and spelling out "thank you". Also I have a great recipe for a sugar cookie. When I was in the 9th grade the home ec. teacher had us use this recipe from the Betty Crocker cookie book and I have been using it ever since, 30+ years. The only thing I changed was from vanilla extract to almond extract.
Ethel's Sugar Cookies
3/4 cup shortening (part softened butter or margarine)
1 cup sugar
2 eggs
1/2 teaspoon almond or vanilla exract
2 1/2 cups all-purpose flour
1 teaspoon baking powder
1 teaspoon salt
Mix shortening, sugar, eggs, and extract thoroughly. Blend in flour, baking powder and salt. Chill at least 1 hour. Heat oven to 400. Roll dough 1/8 inch thick on lightly floured counter. Cut with cookie cutters and place on ungreased baking sheet. Bake 6-8 minutes, or until cookies are a delicate golden color. Let cool slightly on baking sheet before removing to cooling racks.
If decorating with colored sugars, decorate before baking, otherwise frost when completly cooled.