It's best to buy them already paired. The problem is that all clowns start out being male. When they pair up, the larger one becomes female. Whenever you have a pair of clownfish, the larger one will always be the female. Once they become female, they don't go back...
So the risk is that if you buy two random clownfish, you may be buying 2 females and not even know it. If either of them has ever been paired in the past, the larger one would have become the female.