Surely it's his job to find one and refer you?
And a good neurologist should be able to diagnose likelyParkinsons without a Dat scan.
Usually, if the diagnosis is likely, one is prescribed Parkinsons meds and if the symptoms improve, it's Parkinsons.
Also you need to check if the scan is covered under your insurance as I assume it is an expensive procedure.
My first reaction, not knowing any of the people involved, would be to find a new neurologist!