Here is a page with downloadable copies of old Wargames Research Group rules. These are on the WRG website and are offered by the publishers free. There is also the history of the Wargames research group. Includes the older Ancients rules, World War II, and Horse and Musket eras. The re are also downloads of their popular DBA ancients rules. Find them here:
Some of these rule sets are long out of print.