For Ancient History Primary Sources, look for
One of the best ways to find primary sources is to look on the Wikipedia page for your topic. Looking in the Footnotes and Works Cited sections are likely to list primary sources and possibly provide links to them as well!
Because of copyright law, most translations scanned and put on the internet are over 75 years old. As with any discipline, translations build on each other and a more modern translation may be much better, clearer, and easier to read than an older one. Because of this, it’s in your interest to get books with more modern translations rather than read everything on the internet. The two main ones are
The Loeb Classical Library: over 100 volumes of translations of classical sources. Do a search on the author's name, such as Dio Cassius, and look for Loeb Classical Library after the author's name. These are in the library and available for checkout. They all have slightly different call numbers but are generally in the PA’s. This collection is more inclusive than anything listed below for web sources, so more obscure writers should be found here. Make sure to check the spelling of the writer in secondary sources.
The Cambridge Ancient History, ed. by J. B. Bury, S. A. Cook, and F. E. Adcock.New York: Macmillan, 1924-39. 2nd ed. 12 v Call #: REF D57 .C25 1924 (Library has a circulating copy of vols. 1-11 under call number D57 .C25 1924)