If you don't have key limes available, you can use Persian limes (regular limes). It will be delicious, but different. Key limes are smaller, more yellow, and have a bright, distinctive flavor.
You will need about 9-10 key limes for this recipe.
If you don't have a food processor, put your graham crackers in a bag and whack them with a rolling pin or frying pan. Melt the butter and mix it with the crumbs and sugar in a small bowl and press it into a tart shell.