How do I get another clownfish?

So my occ clownfish is doing fine and when I picked him or her up they were with 3 other clowns. Now that I think about it I feel bad for the clown and I was gonna go pick up another clown. I have the appropriate space for them and I was wondering on what I should do. I know I have to get another occ but what size should I get the new one at?