When I was a 'tween, the local 7-11 (2 blocks walk from school) sold packs of 3 backstock comics for 29 cents. Usually there were no DCs but plenty of Marvel and Charlton books. I always looked for the sets without the Charltons, but still ended up with a moderate sized Charlton collection due to the middle book!
My best finds included a Conan 1, Avengers Annual 2, Silver Surfer 1, and ultimately a near-complete set of Captain America from 102-127. Strangely, issues 121 and 123 were the hardest for me to find, but issues 102, 105, and 122 were everywhere. Silver Surfer 1 was also a plentiful book - wish I had bought more than 1! It was one of my favorite books, and I literally read the cover off it.
There was also a corner grocery store that traded kids comics for coupons - if you brought in cutout coupons from the newspaper and the coupons added up to 12 or 15 cents, you got a comic from the backstock. My friend lived around the corner from that store, and he always got better books than I did, because I only got to go about once a month.