I don't think I saw anywhere in either Tokyo Disneyland or DisneySea which sold the American style mouse caps.
They do sell plenty of different headbands (including those with Mickey and Minnie Ears) and "full" hats, but not the American style mouse caps with just the bit that covers your head and no brim.
And I don't recall seeing any personalised headbands or hats, but it's highly possible that any sign advertising a service for writing in Japanese would itself be in Japanese so I wouldn't have understood what it said.
Just got back from TDR and no, there aren't any mouse ear hats, only headbands . Our experience has been that none of the non-US parks carry the ear hats, only headbands.
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