I haven't played the game in years...since crystal came out so I don't
remember the towns name anymore but Ditto appears
to the right of the town where you fight Koga, in the grassy area where there's all those bird trainer people
um...
Pallet town
Pewter city
Veridian City
Vermillion City
Fuscia city (spelling)
...nope can't remember the rest
it's bellow the town with the ghost tower and to the right
of the city where you fight the gym leader Koga yeah that's where
Ditto appear