As I said, there's no one guide to what constitutes "old school," but it's worth checking out Matthew Finch's Quick Primer for Old School Gaming. By the standards it lays out, some of the games I discuss aren't old school, but whatever. Let a thousand flowers bloom. Finch's retroclone, Swords and Wizardry, is free, so you might as well check it out. Honestly, if you're interested in old-school games, it isn't hard to find free stuff online, because for many people this is a labour of love, not a business, and there are lots of great blogs as well. In fact, there are too many to name, so let me just single out Jeff's Gameblog and Middenmurk as excellent ones, with very distinctive, individual takes on old school gaming. Not so much introductory blogs as two examples of what people do with the material.
Marvel Super Heroes material is relatively easy to find on eBay and elsewhere. Indeed, a less scrupulously law-abiding man than I might suggest that practically everything ever written for the game is also easily available online without having to go to dodgy torrent sites and no one seems to care, possibly because TSR is dead and gone and Marvel's got bigger problems, like where to put all their money.
Call of Cthulhu is, of course, easy to find in print and in PDF, and if you don't have it, you're really missing out on one of the great RPGs.