Graviton shields have more capacity at the same class level than either Positron or Molecular shields, but each type of shield has advantages and disadvantages against certain weapon types. Some people like to acquire all 3 types and switch them depending on who's going to be shooting at them, but that can get kind of tedious... and since you've got so many different factions gunning for you, with different weapon types, well, maybe you just want to go for the most capacity.

Gas Miner Naha in Sigma-13 has c7 Graviton shields, and it lies along the popular New Berlin to New Tokyo trade route.

Thank you very much