Epicurus is said to have originally written over 300 works on various subjects, but the vast majority of these writings have been lost.
1) Diogenes Laertius - A major source for Epicurean doctrine is Diogenes Laertius, a Greek biographer (3rd century CE). Lives and Opinions of Eminent Philosophers Book 10 contains three of Epicurus’ letters to his disciples: the "Letter to Herodotus" on physical theory, the "Letter to Menoeceus" on Epicurean ethics, and the "Letter to Pythocles" on astronomical and meteorological matters. Also contained in this book are the “Principal Doctrines” (Kuriai Doxai) in which Diogenes quotes a collection of short sayings excerpted from the writings of Epicurus. Diogenes also fills in topics not covered in the Letters, and provides a list of Epicurus’ writings and other biographical information.
Principal Doctrines, by Epicurus
Letter to Menoikos, by Epicurus
Diogenes Laertius, Lives of Eminent Philosophers, BOOK X, EPICURUS (341-271 B.C.)
Diogenes Laertius: Principal Doctrines of Epicurus
2) Lucretius - wrote the philosophical Latin poem "On the Nature of Things" (De rerum natura) in six books of hexameter verse (first century BCE). This poem covers Epicurean physics and natural philosophy.
The Internet Classics Archive | On the Nature of Things by Lucretius
3) Diogenes of Oenoanda - erected a large inscription on a stone wall in the city of Oenoanda (second century CE, in what is now modern day southwestern Turkey). The Greek inscription contains the basic tenets of Epicureanism, including a treatise on ethics. To this day it is only partially excavated, and the work to reconstruct stone fragments continues.
4) Herculaneum scrolls - Several works of Epicurus, including parts of his major treatise, On Nature have been recovered in damaged condition from the library of a villa in the town of Herculaneum. The papyri rolls were buried and preserved in the ash from the eruption of Mt. Vesuvius in 79 CE. They were rediscovered in 1752, but early attempts to read the scrolls proved difficult and unrolling destroyed the papyri. Scrolls are now being virtually read by newly created technology.
5) Philodemus - The Herculaneum library also held the writings of Philodemus, an Epicurean philosopher in the first century BCE. Philodemus' writings both preseved Epicurean philosophy and also dealt with ongoing developments within Epicurean communities. New editions and translations continue to come forward.
6) The "Vatican Sayings" - a collection of short sayings which partially overlap with those from Diogenes Laertius. Some of these maxims might not be attributable to Epicurus. This 14th century manuscript was rediscovered in 1888.
7) Cicero - presented and also criticized Epicurus’ ideas, especially concerning ethics, in several of his philosophical works, including "On Moral Ends" (De finibus) and the "Tusculan Disputations".
Plutarch, Sextus Empiricus, and the Greek commentators on Aristotle - these writings contain short citations of Epicurus’ works, however they are often taken out of context or presented in a polemical and distorted fashion.
References:
https://plato.stanford.edu/entries/epicurus/
Diogenes of Oenoanda - Wikipedia