Copleston generally follows the accepted methods of citing Greek and Roman authors; these are found in the footnote information he provides.
For example, he will cite the critical edition of Aristotle by its Bekker number (for example, Metaph A 987b 1-3) rather than a page number from a particular edition or translation. These standard "page" references are usually provided along with any scholarly translation, usually to the side of the text.
For many classical authors, the complete original text can be found at Perseus Digital Library, which provides a handy look up function on the same page as the text:
Greek and Roman Materials
Places such as the IPL or Open Library will usually have translations of the major philosophers' works (with standard references); however, the regrettable US copyright law (thanks to Mickey Mouse) often prevents the more current translations from being offered. In any case, one should remember that translations of the same text may differ significantly, especially with philosophical works.