As a followup, I started a thread on Board Game Geek and have found a good number of folks expounding on where they got their games in the 1970s. I also asked the question on a dozen Facebook groups but I won't link to those as there might be some overlap.
Very enterprising. The earliest accounts of where Gary bought games suggest that he favored Brentano's in Chicago to get his Avalon Hill titles in the very early 1960s. My point though was more that Monopoly was a game that could be secured in any number of venues, while, say, to get Metagaming products in the 1970s, if you could buy them over the counter at all, you went to specialty retailers. There are amazing stories of where people went to get copies of D&D in the mid-1970s - I remember Niall Shapero's account in DW of driving around the Bay Area to locate one hobby shop where a copy could be procured.
Still, I'll follow your thread on BGG, I'm interested in what people have to say.