Question: where to catch a ditto in pokemon fire red?
Answer: Routes 13, 14, and 15, Pokemon Mansion (Cinnabar) and the Cerulean Cave. ... 
Cerulean Cave, Pokemon Mansion, and on the route past silence bridge down to Fuschia, Ditto are VERY COMMON in the grass, which requires cut to reach. ... 
If you need more help then you might want to look at an FAQ on this. ... 
At the Cerulean Cave.