Where can you get Key lime juice?
I usually use a brand called Nellie & Joe’s — it’s in a yellowish bottle and is available in the grocery store, Walmart, some liquor stores carry it, or you can get it online.
Besides the answers you’ve already seen, see SAJ’s excellent tip in comment #21.
In my pie, I have just used regular limes I find in the produce section of the grocery store, but SAJ’s “key lime juice” recipe would likely make it even better.