Depends on which island you are on. On Oahu, Star market or Kilgos perhaps. On the Big Island Hilo side, some of your choices would be Garden Exchange and Paradise Plants.

I think you can also get seeds online from the CTAHR page (U of H's College or Tropical Agriculture and Human Resources)which is at: http://www.ctahr.hawaii.edu/seed/ They will sell seed in either bulk or home garden packets.

Sometimes Choi Sum and Ung Choy are sold as entire vegetables with the root intact, aren't they? You could buy some at the grocery store, cut off the tops and plant the roots and see if they would continue to grow. Lots of things will do that, even onion and carrot tops.

Here is a link that might be useful: U of H College of Tropical Agriculture seed sales

I'm on Oahu. I'll try Kilgo's this weekend. Thanks.
Since I'm mainly a hydro grower starter plants works well for me.