Where to get a list of 2005 Peterbilt flash codes?

The service center of any Peterbilt dealership can provide those. You're really better off just to get an SAE J1708 compatible code reader, though... I use a NexIQ ProLink... less expensive ones are available. Flash codes are often very vague... it might tell you, for example, that there's a low coolant warning, but not whether there's a fault in the coolant sensor circuit or not.