GREAT QUESTION!
Let's seenow...
Superman, Batman, Captain America all from The Great Comic Book Heroes. As has been noted a fine collection from 1965.
Amazing Fantasy 15, Fantastic Four 1, Detective 327, Green Lantern 76, Adventure 247, Avengers 4, Brave & Bold 28, Avengers 16, X-Men 1, Action 252 all bought new from local drug stores and convenience stores. Journey Into Mystery 83 used from another kid a few years latter for 25. cents. Like many I too can remember where each of these comics (bought new) were purchased. Wonderful memories!!