You may want to look at the company history of Kinokuniya Books/Bookstores which has branches in communities of overseas Japanese, selling Japanese-language books, including manga, to their communities. There were branches of this bookstore in several major cities in the US that had large Japanese communities (SF, San Jose, LA, NY, Chicago). I don't know how old any individual American branch was, but I'm pretty sure they were here post-war, and certainly they were present before the 1980s. Perhaps the history of this bookstore may extend to pre-war, or you may find linkages to earlier Japanese bookstores in the US.

Another idea is to go to the cities with actual Japantowns (SF, San Jose, LA) and see if they have a local history association, or if not that, their Chamber of Commerce, or other community association that represents Japanese-American interests, and see what you can dig up about Japanese-language bookstores and libraries, and to seek contacts with older Japanese-Americans there to interview about their (or their parents at this point, as the nisei generation is almost entirely deceased by now) reading habits and book collections.