I'm new here, so I couldn't answer earlier: There's a Korean (?) shop - called "Asian Food and Gifts", I think - at
Bebelstr.56 . Phone (Landline): 0711. 63 20 89. The shop is between Spittastrasse and Claudiusstrasse. Public transport stop is "Spittastrasse". Take the U9 (towards Enstation Vogelsang), or the U4 (Terminus Botnang) from downtown Stuttgart
I often saw a sign in their window saying something like "Samstag Sushi" (Sushi on Saturdays).
This is a Korean store, i think, with food mainly from Korea, Taiwan, and China, IIRC ... I'm not very much interested in Asian food or so, but their "Instant Noodles with Beef Flavor" are addictice (well, nearly ... :) if you add 1 or 2 spoonful of vinegar ... :) ... I'd go there and have a look: If they sell complete sushi, they might be offering the ingredients, seasonings etc. as well ..
I remember even having bought fresh (?) Soybean sprouts years ago. I think they had them once or twice a week.
Prices were at a very reasonable level last time I was there (about 2 months or so ..)
There are/were even more Asian shops in Stuttgart, I just don't have the exact locations/addresses of them ... let me know if you need them ...
Good luck