Just about to go to bed, I just HAD to type in "buy Japanese silk" into Google after reading mentions of it recently in Mary Corbet's blog. My Google is set to Australian pages, so might come up with different results to other Googles.
http://www.flickr.com/groups/33556709@N00/discuss/72157594206565570/
has heaps of links (some dead) to various shops all over the world (a lot in Etsy - great place!) of sellers of Japanese silk.
The best I've seen so far (only gotten so far in the list) is Purl Bee. I'm seeing a lot of Japanese silk, but I want plain, not printed! They have heaps of colours!
I am making the assumption here that any Japanese silk is suitable for embroidery. I don’t know – I’d ask of a particular seller first.
Enjoy, and G'nite!