On this page of the guide, you will find information about the location of Bob's jacket in Mr. Prepper.
Bob's a merchant from the Old Creek mine. When you fix the elevator for him and meet him in the mine at level -1, he'll give you the job of finding his old jacket. This is one of the story quests in Mr. Prepper. To find Bob's jacket, you must go to level -3. Before that, however, you need to obtain an access card by exploring level -2. At level -3, you will find a jacket, not an access card to level -4. The jacket's location will be random because it is generated anew when you start a new game.