So, where does Grandma work?
According to Tsuritama Wikipedia, the store Grandma Keito works at is the
Samuel Cocking Garden


  Kate (ケイト Keito)
  
  Yuki's grandmother. She's French, and has a warm
  and cheerful personality. She allowed Haru to live with her and Yuki.
  She often gives Haru advice concerning his relationship with Yuki and
  human social interactions. She works at the Samuel Cocking Garden in
  Enoshima.


This garden, also known as the Enoshima Tropical Plants Garden, is an actual botanical garden in Enoshima, Japan