Usually you would find Bug Catchers in Eterna Forest. That's the first place I would check.
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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.
its a cascoon
You are able to get Dustox by evolving Cascoon. Wurmple-Silcoon-Beautifly Wurmple-Cascoon-Dustox
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.