As you know, the original Dungeons & Dragons game has long been out of print. Even if you can’t get your hands on one of the older rule sets, you can still use the free substitute Basic Fantasy (initially released in 2006). For those of you who prefer AD&D 1st edition, you can try out OSRIC (also initially released in 2006).
Some of the contributing authors here at Heroes Only have contributed to the Basic Fantasy collection and can attest to the openness and community spirit of the project. Chris Gonnerman, the original creator and editor of the game materials publishes all the work online in both source and PDF format.
If you want to get a hard copy, you can also buy the latest editions available in paperback or hardback.