For 1965 (64-1/2) Mustangs with 289, A/C and Generators, the 1960-68 Ford Master Parts Catalog (MPC) shows that the idler pulley assembly, either with or without PS, is Part # C3AZ-8678-B.
This same idler pulley assembly is also shown to be used on 1963-64 Fairlanes with A/C and 221, 260 and 289 engines, 1960 Falcons with 260 engines, and 1963-64 Full Size Fords with A/C and 260 and 289 engines. The pulley on the idler assembly is cast iron.
This should provide a lot of sources for what you need.
Hope this helps.
Bob