My cousin and I REALLY need this Pokemon badly!!!! We need to know ..

1. You find Ditto on Route 218 using the Poke Radar.

2. You go in the patch of grass near Canalave and use the Poke Radar.

3. After you've SEEN all 150 Pokemon in the Sinnoh Dex, go to Sandgem Town and talk to Rowan. You'll see Oak and he'll give you the National Dex. Now, go to the end of Route 221 [just south of Sandgem] and go in Pal Park. You'll see Oak again and he'll give you the Poke Radar. [NOTE: If you go to his house in Eterna City he'll give you the Dubious Disc. What a giving guy, yeah?]

On Rt 218
Use the pokeradar
After getting the National Dex, Go to Prof Rowan's Lab. Oak will come and upgrade your pokedex to the National Dex and leave. Head to Pal Park on Rt 221 and talk to Oak again. He will give it to you and an app for your poketch. To use it, go to Key Items and select it and use it(easier to use if set to Y). You will see anywhere from 0-4 patches of grass shake. If you walk into one of those patches, you will fight a wild pokemon. If you fight someone before you get to one of the patches that shook, you will not find someone in the shaking patch so use a Repel(Super or Max). If you see a patch shake and change colors, head to that patch. You will encounter a shiny pokemon. From what I have found, if you go to a patch that shakes with white on top, you have a better chance at finding an older pokemon

If you want to get ditto you will need to use the pokeradar in route 218, you can get the pokeradar by seeing every Pokemon in the sinnoh dex and then talking to prof. Rowan in his lab. After you talk to Rowan Prof. Oak will come into the lab and give you the pokeradar. The pokeradar is in key items and you can only use it in grass, when you activate it, there will bee patches of grass shaking about just walk in one and you will find a pokemon.

P.S. Not every time you use the pokeradar in route 218 will you find Ditto

Hope I've helped
Diamond281294

You get it on route 218 and you need the pokeradar.
Once you beat the elite four you can get ditto.
You get the pokerader from Oak after you beat the elite four but I don't know how to use it though.

1. You get ditto in fire red or leaf green; I don't remember where.
2. You can just catch a ditto anywhere, just like a regular pokemon.
3. You have to complete the sinnoh dex, then go talk to Prof. What's-his-name (the sinnoh prof) and then the kanto prof will show up, upgrade your dex, and leave. After that, the sinnoh prof will give you the poke radar as a gift.

You get Ditto outside Canalave city using the pokeradar you get the pokeradar in sandgem town (I think you might have to have the national dex) you get it from either dawn/lukas or professor rowan/oak