The information for this site is from the city government of Mexico City. As their website is only in the spanish language
I modified and translated it into english. The address of of the original website (in spanish) is:
www.metro.df.gob.mx/index1.html

The most important bus stations in Mexico City are:

• Central Camionera del Norte (Bus Station North)
• Central Camionera del Sur (Bus Station South) also called "Tasqueña".
• Central Camionera del Poniente (Bus Station West)
• Central Camionera del Oriente (Bus Station East) also called "Tapo".

To get to San Pedro Pochutla (11 km from Puerto Angel) as fast as possible
buy your ticket at "Estrella Roja" or "Futura" in the bus station south.

To Oaxaca you go best with "UNO", "ADO", "Cristobal Colón" or "AU" from bus station east (Tapo). The bus line "UNO" has luxury buses where you can sleep comfortably.

Website of "UNO"
Website of "Cristobal Colón"