Trader Jo's has a frozen key lime pie that's pretty darn good. Maybe not the same as fresh, but if you just need a fix it does the trick. Inexpensive and you can top it with your own homemade whip cream if you like.
Two recent samplings: Marmalade in Malibu - great graham cracker crust and perfectly tart. Palace Café in Santa Barbara (okay, so it's not LA - you didn't specify) was also very good, although a little sweeter than I would have liked.