You can meet a ninja at a theme park.
I am sorry to break your dream. In a fact, Ninja appearing in manga or anime etc is fiction. The role of a ninja was not a warrior but a spy. Therefore they did not wear ninja costumes. For excample, they disguised themselves as a farmer, and engaged in secret maneuvers at the war or a power struggle. They do not use the magic called ‘ninjutsu’, and had not super natural power.
However, the demand for ninja as entertainment is high, Therefore, we sometimes see a ninja recruitment advertisement. You will may be ablet to get a job as a ninja.
Well, I want to correct it. There is a village of ninja somewhere in Japan, and ninja tribe may live in secret.