I can recommend "The Stoic Reader" by Brad Inwood. It contains some of the most important sources for Greek Stoicism. Another great collection of sources is "Hellenistic philosophers", volume 1 by Anthony Long and David Sedley. Cicero's philosophical writings are also important but since he is not a Stoic himself and since he has his own agenda he should be read with caution. Seneca is hugely important. In my opinion, he is the single most important source since he is a dedicated Stoic himself and offers in depth treatments of an incredible amount of topics in Stoicism. Obviously, Epictetus and Marcus Aurelius are also important but, in my opinion, Cicero and Seneca goes into much greater depth.