Wargames Research Group has a page with lots of their older sets available, including the Fifth Edition Ancients, which were the standard for many years.
WRG
WRG 1925 – 1950 Armor and Infantry Rules
The Wayback machine has stored a copy of the WRG 1925 to 1950 armor and infantry rules from 1973.
WRG Ancients 7th Edition
WGR Ancients 7th Edition is available for free as a pdf download.