It can be anywhere. usually it is silkoon but after you get out of the eterna forest and walk in the grass above the fishermen on the dock there is a lot of cascoon.
Chat with our AI personalities
You can find Cascoon in Pokemon Platinum at Route 205 North, Eterna Forest.
There is a TON in Eterna Forest. Go there and run around and you should see one within a minute.
Usually you would find Bug Catchers in Eterna Forest. That's the first place I would check.
Catch a female wurmple Or if you have Pokemon ruby (gameboy game) then just plug that in to your DS when you play Pokemon platinum. Go to route 205 Eterna Forest and cascoon will have an increased chance of appearing.
from this green haired lady in eterna forest after you help get out of the forest to eterna city