For visitors, the museum shop would be one of the "must-go" places in Ghibli Museum.
Some people, who couldn't get the tickets, may think if they can visit the museum shop without tickets.
Unfortunately, the museum shop is inside the museum entrance, and you cannot visit the shop only without tickets.
If you want to see and buy Ghibli goods, we would recommend you to visit "Donguri Kyowakoku" store as an alternative option.
They have around 40 stores in Japan.
Below are list of some stores in Tokyo.
Donguri Kyowakoku @ Tokyo station - Google map
Donguri Kyowakoku @ Ikebukuro - Google map
Donguri Kyowakoku @ Tokyo Skytree - Google map
Donguri Kyowakoku @ Venus fort Odaiba - Google map