either under the truck like everyone thought it would be (seriously, strenght on the truck? lol...)
maybe in cinabar, there is that creepy house with info about mew and mewtwo.. i think it should belong there...
wasn't that place where they made mewtwo? so mew should appear where mewtwo was because how he was clonned there
or a secret area on the museum, you have to talk with the dude with mewtwo in the party (or as first pokemon)
a new scientist jumps out of joy seing the mewtwo, put some bullshit story about how they found a pokemon never seen, however it's either sleeping/a fossil.
the scientist shows you the secret room, team rocket appears, fight them, then you can revive mew from either sleep or fossil...
btw, IIRC there is already an island with mew on that game which you need a ticket..
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PS: or when ash ketchup did found mew on the anime? i don't remember if he saw him on a router or some special place..
either way, make sure it's a secret or a new area which requires something..
i remember the first time i played pokemon yellow as soon as i got surf i tried to fight zapdos lel, i got pretty much pwoned... i didn't knew zapdos back then.. it was a really surprise to find that bird there
PPS: ketchup's ash is a disgusting powder