There were a lot of military surplus hp compressors available cheap at that time. Ingersoll Rand 3321's with Wisconsin 4 cylinder engines were a popular item, and must have been readily available. I used to see a fair number around here into the 70's still in use. Coupled with just a 5 hp electric engine you could get a good 5 or 6 cfm at 2250 psi in a fairly quiet running package. I still have one of these that I acquired in 1970, and used in the dive shop until 1976, when I acquired a larger compressor. Clubs often had them on trailers. Luckily for me in the late 50's and early 60's there was a guy with one in my small town, so air fills were readily available for me.
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The miniature surplus Cornelius and Kidde compressors were very popular with individuals, and lots of them were in use due to very low price and large supply.
I hear stories of people getting their tanks filled at fire departments in the early days. That must have been fairly common in the 50's and 60's.