I would consider one that is ready when it is at least 6 months old and proven stable. As well as having the aquarium keeper experienced as well at least a year would be good. Before purchase one should study and ask questions of the type they decide on to see if it is right for them, and or, their choice of clown fish to see if they are compatable. They should not be housed in nano tanks either or at least 29/30 gallons minimum IMO. If purchasing either an anemone or a clown fish you should also purchase the book Clownfishes by Joyce Wilkerson.
Just a few opinions.
Thomas