Hi,
All the major car hire companies have a depot near the ferry terminal at Devonport. That is - Budget, Europcar, Thrifty, Hertz and Avis. Not sure if any of the smaller and possibly cheaper companies do. So I would suggest getting online quotes from each of those and then booking for pickup at the Devonport ferry terminal location (the Esplanade, East Devonport).
All the best for your trip!
Jo