The bookstore at the Wheaton Library in Maryland sells National Geographics for 25 cents each (20 cents for 5 or more) back to 1950. Before 1950 they are 50 cents each, but those issues are in fair to poor condition. People donate their NG collections all the time, but the library has to "recycle" (i.e. throw them away) every few months because they don’t have the shelf space to store all the donations. However, they don’t get many donations of pre-1960 NG's.
No, they won't mail them to you. But here's their link:
Why am I telling you this? Because I see so many posts of people wanting to sell their "complete" collection from "1975 to the present." But the cost of shipping is more than the magazines are worth. And at these prices, it makes more sense to buy them used than to get a yearly subscription ($2.40 vs $15.00).
The bottom line is that your NG's (1950 - present) are worth more as a tax-deduction, if your library or school will take them. If they won't, that should tell you something!