If you are in America, you are going to have to go to an import shop. Most of the import shops I have seen are run by Koreans, not by Japanese, but they sell both Korean and Japanese things. There is one close to my house here. It is in Del City, across the street from Wal-Mart. It is called Ko-Mart. We really like mochi, and when we get to feeling we want some, we know they will have some. They also sell Japanese rice crackers, seaweed, and all kinds of good things. They also have the best rice in town.
When I lived in Texas, we used to go to a Korean run import shop in Ft. Worth. They also had Japanese and Korean things both like the one here. Like now, we used to go there to find mochi. Japanese are less likely to stay in America, but the Koreans stay, and when they open up shops, they sell both Korean and Japanese things. I am not sure where you are in America, but look for a Korean import shop close to you because they also sell Japanese things.