Hello I live in Michigan as well. Where in Michigan do you live? I'm trying to grow vegetables myself via container
gardening.

As far as a good retailer that offers a wide variety of Asian vegetable seeds and are reputable I don't of any stores in Michigan that really stock a decent variety of the seed varieties you're looking for.

However I do know of an online store that specializes exclusively in the sales of asian vegetable seeds.

I will give you the link. The name of the seed retailer
is Kitazawa Seed.

I know you can grow daikon radish in Michigan; it matures very quickly (about a month to 45 days depending on the variety you get). To my knowledge it's also called icicle radish and if you look hard enough you should be able to find a seed store or hardware store that carries this.

White radish likes cool weather and hates hot weather.
If it becomes too hot it will bolt which essentially means it will start to create flowers. This will make the radish pithy and very spicy.

Unfortunately regarding the other varieties of vegetables
you want to grow, I can't really help you there as I've never grown them myself.

However here is the link to the seed company I was talking about:

http://www.kitazawaseed.com/

Check out Evergreen Seeds as well, link below.

Bok choy is really easy to grow and can be used in 30 days or less from my experience. Maybe you have time for a fall planting?

Here is a link that might be useful: Evergreen Seeds

I have had good experience with Evergreen Seeds as well.

They have many types of Asian Brassicas/ Cabbages Bok choy, Pak Choy etc .. They do best planted in August or early September, in Michigan it tends to go from April frosts to Too - Hot - for Bok Choy in relatively few weeks, so fall is genreally best for them.

Evergreen has Bitter Melon from several countries and they grow well, if you get them off to a warm start ( may need to cover til early - mid June ) These like 80+ F and lots of water. I grow them on a 7 - 8' trellis.

Perilla can become invasive if you let it go to seed. This also seems to grow better if planted in summer ( July ) and grows well in September til frost.

Japanese and Korean seeds do well for us in Michigan as well, we have similar climates ( especially August - November )

HTH Nam

Seed companies I have done business with and would recommend that carry at least some Asian veggies are:

Territorial Seed
Johnny's
Burpee

Just check the FAQ for this forum for Asian Vegetable Seed Sources.

Here is a link that might be useful: Asian Vegetable Forum FAQ