Primary sources in Classics are the literary works (poems and plays for example, histories, documents (private and public), and artifacts (pottery, coins and sculptures, for example) created by the ancient Greeks and Romans.
The Loeb Classical Library is a respected modern collection of Greek and Roman literature. Each volume contains the Greek or Latin text on one page with the English translation on the opposite page. To locate a Loeb edition in the NEOS catalogue try a keyword search with the name of an ancient author and the word Loeb. For example, Plutarch and Loeb.
Penguin and Oxford (World's Classics) also publish good translations of the most popular ancient authors and their well know works. In the NEOS catalgoue try a keyword search with the name of an ancient author and the word penguin or oxford world's classics. For example, thucydides and penguin.
When trying to locate primary sources for your research think about the ancient authors you have studied in class, read your textbook, class notes and examine your course outline.