Key Limes are indigenous to southern Florida, especially Key West. Unlike regular limes, they are smaller, juicier, and have a yellow skin. Even though the outside is yellow, the inside is green. They have a distinct flavor, as they are more tart and acidic. If you are unable to travel to south Florida for a key lime, you might find them in your local Latin grocery store. Bottled Key lime juice can be ordered from directly from Florida. Nellie and Joe's Famous Key West Lime Juice is carried in my regular supermarket. I've gotten good results from their brand.
Key Lime Pie recipes have few variations. The most common variations are the topping and the crust. The toppings are either a meringue or whipped cream. Using a meringue topping has two advantages. The first is economical. Most Key Lime pies fillings call for egg yolks. The leftover egg whites can be used to make the meringue. The second advantage is meringue always makes a beautiful presentation, those lovely peaks broiled slightly to a golden brown. Whipped cream works for me, because I don't like meringue. It is also easier to make whipped cream, because there is little room for error unlike the meringue. Traditionally, a regular piecrust is used, but a graham cracker crust is becoming more popular. Since a Key Lime Pie is kept and served cold, the regular crust tends to get soggy, but a graham cracker crust does not. Puritans will argue that the graham cracker crust takes away some of the flavor the Key Lime Pie filling. I'll leave the choice up to you.