Candy heart sprinkles are usually on shelves in grocery stores year-round in the baking aisle next to the other sprinkles. If you can't seem to find them then you can substitute with another heart-shaped candy or easily draw the hearts on yourself with red decorator gel (or pipe them on with red tinted frosting). If all else fails, you can dot the top of the cookies with red M&M's and still get that festive Grinch look.