i've seen giants from different parts of the world, like Brazil, and here in the US. I believe they all have the potential. you just have to breed and raise them to be champion quality. give the parents optimal care. feed the fry all they can eat. cull all the small and not perfect ones. continue giving optimal care and as much quality food as they can eat, and keep culling the non perfect ones.
Genetics play a big role yes. But so do care and diet. Grooming goldfish is a subsection of the hobby in itself.
Plenty of good lines available outside Asia. But a $10 Petco oranda isn't going to win any shows either. Finding quality stock is key.
But I get the impression that you're asking in hopes that they DON'T get big. If that's what you're hoping for, a chain store cheap oranda is your best bet. Those will rarely beat 6-7 inches.
Edit: just noticed your username. If you're going for a good grow out, you want Asian stock for sure, or one of a select handful of north American breeders. I prefer Chinese fish myself. But Thai and Japanese produce quality as well.