Since one of the other two Tokyo shops is in Odaiba and the other is in the Western suburbs, the Shinagawa shop seems to be the most convenient. It's very close to Shinagawa station on the Yamanote "loop" line.

Does seem that the store in Haneda (which is very small) is the only one dedicated to this character. The selection of Shirotan branded items at Shinagawa might be disappointing. Am pretty sure I bought a kinetic ham eating Ponyo bath toy at Shirotan Town in Haneda, so even here there wasn't enough white seal produce to solely stock the shop.