Offenhauser still makes them - look on the summit or jegs web site. It is a single plane with short runners so can take some tinkering to run right on the street.
You will not find Offy dual quad SBM intakes in the Jegs or Summit catalog. They are made-to-order and are sold only by Phethean Racing Products, AFAIK. They are calling them 360° Equa-Flow manifolds. These appear to be of dual plane design.
Chrysler: 273-318 V8 1966 and later Dual Quad Equa-Flow (5615) $516.15
Chrysler: 273-318 V8 1966 and later Dual Quad Hi-Rise (5698) $598.73
