Unfortunately, less than one percent of the original Stoic writings have survived. The most important sources we have today are:
Seneca’s Letters, Essays, and Dialogues: Seneca was a Roman statesman and advisor to Emperor Nero
Epictetus’ Handbook and Discourses: Compiled by his student Arrian, Epictetus was a Greek ex-slave and the only Stoic teacher whose thoughts survive in book form
Marcus Aurelius’ Meditations: A private Stoic diary of the Roman emperor, deeply influenced by Epictetus