1. Go to Slateport City: Travel back to Slateport City where you first obtained the Devon Scope from Captain Stern.
2. Visit the Weather Institute: Enter the Weather Institute building, which is near the Slateport Gym.
3. Find Weather Guru Webster: Inside the Weather Institute, locate Weather Guru Webster. He is a NPC characterized by glasses and blue clothing.
4. Inform Webster: Speak with Weather Guru Webster and inform him that you lost the Devon Scope.
5. Retrieve It: After reporting the lost Devon Scope, he'll retrieve it for you through his influence and return it to you. You will get your lost Devon Scope back immediately.