I don't care for limes, so I go for lemon. And you?
I have made it with fresh whole Key limes, I bought a bottle of Key Lime juice and made once, but more often I have made it with real lime juice from a bottle and few fresh ones with a bit of grated rind, adding a 1/3 cup of lemon juice balances off the flavour, I was in Key West FL and had it, even had mojitos made with KLJ, while it is not authentic it is a reasonable substitute when in a pinch, I make the baked kind with the raw eggs and Eagle brand milk, I have made the non baked version but prefer this one myself.
You can use any limes, but then it would be a Key Lime Pie, it would just be a Lime Pie. There is no real substitute for key limes, they have a very distinct flavor. They are smaller and have skin that is more yellow than our regular green limes.
You can use regular limes, but the flavor will be different- still delicious, though! Key limes, are lighter green, on the inside. If you only have regular limes, use them! It's fine. Let me know, if you need a recipe.
It's best to use the right ingredients for the dish you are making but sometimes it's not possible if you can't find the ingredients to use. So you can go as far as substitute the taste but not the smell or fragrant of the dish like cooking curry. You have so many curry dishes from different ethnic cuisine so you need to get a specify ingredients to bring up the fragrant of the dish to be successful to duplicate the dish. For key lime pie the taste is tangy filling you can substitute with any line even kaffir lime or kalamansi lime if you don't have key lime
