I don't think they sell phyllite powder in America, I have seen it in solid rock form but not in powder form. What we have avaliable are glacial, basalt and azomite powder, which one do you think is can be a substitute to make JS with.
You can create JS without using these. Rock powder and mud can be used, but azomite is not recommended. Azomite has a low specific gravity, so it is very inconvenient to precipitate JS.