>> Do you know if the seed winters over?
It wouldn't surprise me if it reseeds, but I haven't tried it. The plant itself is very cold hardy (or, at least, many cultivars advertise that). Someone might have let it keep going through several frosts to get some fall harvests, and then forgotten it. If it's like several other Brassicas that I "let go", it might go to seed in very early spring and then drop a TON of seeds.
I don't know how different various tatsoi varieties are from each orher, but here are a few sources:
With Kitazawa, you just have to know to ask for the small packet, which is usually $2 for a "scoop", which is often 2 grams.
Kitazawa Seeds - regular green tatsoi
http://www.kitazawaseed.com/se...
Kitazawa Seeds - savoyed green tatsoi
http://www.kitazawaseed.com/se...
Kitazawa Seeds - red-violet tatsoi
http://www.kitazawaseed.com/se...
Evergreen Seeds
http://www.evergreenseeds.com/... (Tah Tsai Tatsoi)
http://www.evergreenseeds.com/... (Korean Red Tatsoi)
Cherry Gal
http://www.cherrygal.com/asian...
UK: Nicky's Nursery
http://www.nickys-nursery.co.u...