I agree with Freak, unless you are willing to risk fighting and death, buy one, or buy a pair that has already been established in the store or in someone else's tank.
Any and all clowns are considered somewhat aggressive, but all individual fish are unique. Tomato clowns are considered some of the most aggressive clowns too, and I have a pair of tomato clowns that are like the fuzzy puppies of my tank.
A clown will not only be aggressive towards its own kind, but towards other fish too. You may have to decide what other fish you want in the tank based on the aggression level of the clown.