Comment You don't even need to buy them with Retroclones (Score 1) 36
There are plenty of "retroclones" out there, like OSRIC, Labyrinth Lord, Basic Fantasy, Dark Dungeons, etc. are just re-implementations of the various OD&D, 1st/2nd edition, Rules Cyclopedia, etc. rulesets, all downloadable.
I'm not aware of the newer stuff (3rd ed or newer, I guess they are not 'retro' enough.), but the older stuff is good enough to get the framework for interesting play.
There's also Dungeon Slayers, whose basic rules are downloadable.