Where do you find Mew?

Solution 1:

Mew, along with the other legendaries, cannot be seen until you've completed the game so even if you've explored every inch of every course, you will have to replay them after the story is done if you want to complete the photodex.

Mew can be found in

Founja Jungle at night.

There's only so many things you can do, so that should be enough info to get you started, but if you still can't find him after a few tries:

when you enter the course, immediately turn around and play the melody

and if you want the four-star photo

hit Mew's bubble with a fluffruit three times, then play the melody at the ruins after the cliff. Mew should appear and allow you to snap a photo