If you want a male or female you should wait til their about 4in. Better chance in determining gender. You can get flowerhorns from petco/smart but usually 7 out of 10 are low grade. I would stick with special mom and pop fish stores imho.
The best ones are opinions. I personally don't like a Flowerhorn with an obnoxious kok where it looks like a galaxy. I like a medium to large with pearls and good markings.
Go to aquabid.com and check out their Flowerhorn section. They're some very nice fish in there. If you want a "masterpiece" it'll cost you.
In showing you pics from master piece to large to medium kok