I just need to find where (If you can) Gastly or it's evolutions.
Gastly, Haunter, or Gengar cannot be found in ORAS.
To get it, you will have to trade.
If you have another 3DS and Y/X, you can find Haunter on Route 14, Route 19, Victory Road, and Frost Cavern.
Hope I helped!
In order to get Ghastly in ORAS, you need to breed a Haunter or Gengar in Pokemon X and Y, then you can trade it over to ORAS because you are unable to catch Ghastly or any of it's evolutions in ORAS.