There are these posters of a similar size:
https://www.ebay.com/itm/30419821768...0AAOSwqclgg1pT
https://www.ebay.com/itm/11455077711...UAAOSwY~laKGlm
Circa 1966-67 produced by Personality Posters out of NY
The one with heavy wear has a 1966 copyright, and the one from the hippie head shop has a 1967 copyright.
I can’t speak for any sort of stadium posters, but found these vintage examples interesting.