Classical D&D -- Obtaining the Rules
TSR kept the classical version of D&D in print -- in one edition or another -- from 1981 to the mid-1990s. While they are now out of print, copies are easy to get (and often very affordable) on eBay -- click here for more information on buying Classical D&D items on eBay. Copies in PDF format are also available for US$5 or US$6 a book or set -- except for the 1981 Basic/Expert sets, unfortunately.
Fan Rules Restatements
Fan "restatements" of older editions of D&D using the Open Gaming License and D&D SRD from Wizards of the Coast have become fairly popular in recent years. Two versions of the classical D&D rules are currently available in a "restatement/simulacrum" edition: Basic Fantasy Roleplaying and Labyrinth Lord. Both seem to be based on the B/X version of the rules. While they both have minor variations from the original TSR rules, both are so close that it is a no-brainer to run published classical D&D adventures with them. Both are available as PDF files for free or as low-cost print-on-demand version.
What Rules Do I Really Need?
The classical version of D&D has a number of rulebooks -- in several editions -- which can make getting started a bit confusing.
- The least expansive way to go is to just buy a copy of the Rules Cyclopedia. It cheap in pdf format and not very expensive as a used hardcopy and it contains everything needed to play or to run a game except a starter adventure.
- As an alternative you can pick up either the B/X version or the BECMI version of the Basic Set and Expert Set and have all the rules you need for years of adventuring.
- Finally, you can use either of the fan-produced "restatement/simulacrum" editions: Basic Fantasy Roleplaying or Labyrinth Lord. Either can be used with all the Basic and Expert level SD&D modules, although BFRP requires a bit more effort as it changed the way armor class works.
That said, I personally recommend either using the free Labyrinth Lord rules or just picking up the Rules Cyclopedia. Either is an inexpensive solution that is easy to use with all of the published modules. This lets you spend more money on adventures and campaign settings. (I'd pick up a pdf of B1 or B2 to start with and a pdf of GAZ1 to use as a setting.) Total Cost is under $30 and that includes a used copy of the Rules Cyclopedia hardback.