Is this what you're looking for?

If it is then you can mail order seeds from Evergreen or Kitazawa's. I think both places have a minimum order but they offer so many interesting varieties that i don't think you'll have trouble with that ; )

Here is a link that might be useful: Kitazawa Yu Choy

Hmm, close but I don't think the leaves are right. They're roundish tip leaves.

There is a store near dilligham (Sugoi, Kalihi Ko'olau farmers, Youngs Fish Market) called Fukuda Seed store. They sell japanese and local seeds there. I have seen that they have Choy Sum seeds there.