You've almost no chance,, a mated pair is only a mated pair if they're spawning and you're unlikely to see much spawning going on at your LFS. Pairing up clowns is easier with some species than with others,,,, what kind would you like for your tank?
There are a few places that sell mated pairs online,, Aquacon.com is one:
I have both wild-caught and tank-raised pairs of true percula clownfish. Assuming this is the species you see at your LFS, the surest way to form a pair with minimal chance of fighting is to purchase by size. If wild-caught, choose one that is over 2" long, and one that is as small as possible (ie 1"). If they are captive-raised they are more likely to all be about the same size (closer to 1 1/4"), and you can select any two because they are all immature and a pair will result.
Joyce Wilkerson's book "Clownfishes" is very valuable and enjoyable reading if you are interested in clownfish pairs.