Museum shops are ideal places to buy souvenirs in Japan. They offer wide range of products from T-shirt, toys and even foods. Also sell exclusive products which you cannot get them somewhere else.
The difference between Japanese museum shops and European museum shops is definitely that Japanese shops sell foods, especially confectioneries. These confectioneries are sometime exclusive products which sold only at the museums. Therefore if you miss to buy the confectionery at the museum, you have no chance to buy it at supermarket or other gift shop.
Museum shop at Tanegashima Space Center's Museum of Space Science and Technology offers rocket and space related merchandises, including Space foods and Space Water - Uchu no tanemizu.