And I say this as someone who has cursed key limes and swore I would find a substitute that was less work.
They are stubborn little things about half the size of the typical limes (known as "Persian" limes) we get year round in the U.S. but have just as many segments and even more seeds. They are too small for citrus reamers and you need to squeeze a quorum of them to get enough juice for a pie.
But the distinctive grassy tang of authentic key lime juice cannot be substituted. And the reason I wasn't able to find a substitute or bottle of good key lime juice is because there isn't one. Most commercial juice available comes from concentrate.
Since one is rarely presented with a choice when it comes to buying key limes - either the store has them or not, and they will only have one kind (unless you are in the Florida Keys) - you will likely get a mixed bag. It usually takes me 10 to 15 key limes to produce the same amount of juice that just 3 or 4 Persian limes produce.
To make the process less excrutiating, I use the usual citrus tricks. Microwave the limes for 20 seconds and roll them on the counter or cutting board with a firm hand to get those stingy things to give up their juice.
Luckily, the reward for juicing and zesting all those tiny chartreuse golf balls is that key lime pie is very easy to make. The tang needs very little dressing up.
After the jump is a very simple and flexible "recipe" for key lime pie. But you can use the juice for a yummy chicken marinade or Key Lime Bars.
There was a reason I put the word recipe in quotation marks. I make this according to my whim and waistline, so it's a little different every time This is one of the few "semi-homemade" pies (i.e. something not made completely from scratch) that I find completely acceptable and yummy and love to make. Graham cracker crusts are better when homemade, but the difference between sotre-bought and homemade is not as great as regular pie pastry. Once I mastered pie pastry, I never went back.