I think you’ll find the earliest version is this, from 1964:

“The modern conservative is not even especially modern. He is engaged, on the contrary, in one of man’s oldest, best financed, most applauded, and, on the whole, least successful exercises in moral philosophy. That is the search for a superior moral justification for selfishness.”

Galbraith “Let us begin: An invitation to action on poverty”, Congressional record: Proceedings and debates of the 88th Congress, Second session (1964) volume 110, part 3, page 4075.

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