The best key lime juice is Nellie & Joe's. I had stocked up on it when
I went to the Keys some years back, but soon learned that if a bottle
(even unopened) isn't used within a few months, it separates. Might be
usable, but it looked NASTY. To my surprise I did find it in a
supermarket in Hawaii, where even some products common on the Mainland
are difficult to obtain, so perhaps your supermarket has it. If not, you
can get it from cookscorner.com for $2.99 a bottle, plus shipping.

I agree, fresh is best, but you can't always find key limes and it's
nice to have a bottle on hand for that spur-of-the-moment pie.