What kind of parts you looking for? Everything is still available and sometimes are on the shelf for my 54 2 ton. CarQuest, NAPA or any other F.L.A.P.S. that's been around a while should be able to get anything you would need for a 60 - 66 era truck.
Our local powertrain store has several hydro-vac booster styles in stock. We also have a brake shop that can reline the shoes. Several machine shops can sleeve wheel cylinders and the cups are always a stock item at parts stores. The only thing that you may need to order would be a master and hoses.
Scott
