Try Maria's restaurant in the 20 November market for breakfast. Greta people, great food and well priced. The coffe is Oaxacan and served ina bowl with a semi sweet roll, Oaxacan style.
On the Zocolo, La Primevera is excellent as well. Our favorite hang out in the afternoons.
Try contacting Alvin Starkman too. He is a fountain of Oaxacan information.