Rockauto? That accumulator was used on every GM passenger car from 1977 until 1984. It's not an uncommon part (Unless you're not in the USA).
GM NOS probably not going to be easy to find. Most of that stock would have been run through in the 1990s with people doing R134 conversions. But they didn't have any distinct markings on them, so who's going to be able to tell?