I remember a sign "Soy milk" on the Brovki str. Between University of Informatics an Radioelectronics buildings. if I have not mistaken right after the second block wich address is 6-Brovki str., But it was quite long time ago. I've never seen this stuff in shops. The only thing i can think of is here also I know a health meal shop in the Metro Pushkinskaya. May be there you'll find some kind of the stuff, I'm really not sure