Nowadays, people can privately import supplements from overseas without difficulty.
The difference between American and Japanese supplements is that American products are often available as single ingredient products. For instance, they have Vitamin B1-only products.
This is because the environments in which supplements are sold differ between the U.S. and Japan. In the U.S., registered dietitians are working at supplement stores, providing face-to-face consultation to customers visiting the stores, and selling supplements tailored for the customer.
Japan does not have such a system in place. Thus, majority of supplement products usually contain multiple ingredients that are combined for specific purposes (for example, supplements effective for eye strain).