I'm sure someone will correct me if I'm wrong, but I believe a passenger's side mirror was an option for most 60's and 70's B bodies. Classic Industries has RH mirrors that match your factory LH mirror and if it was me, I'd just buy one.
Centerline 64 Dodge Polara 426 Street Wedge - For when I want to go fast 99 Corvette Z-06 - For when I want to turn corners