If you do not want to take a plane, you have an alternative to travel out of Mexico City by bus.
Tickets online available here (it might be a problem with accepting foreign credit cards…):
TAP
Additionally for certain bus lines (ETN) tickets can be bought in physical selling points close do Casa de Chiles:
Insurgentes, Monterrey 6, Int. Local C, Colonia Roma
Frontera 217, Int. Interior Hotel Benidorm, Colonia Roma
Madero 30, Int. Interior Hotel Ritz, Colonia Centro
Otherwise, there are 4 bus terminals in Mexico City, where you can buy all tickets you need:
Mexico Norte, Avenida Cien Metros 4907, Colonia Magdalena de las Salinas
Mexico Poniente, Sur 122, Esquina con Río Tacubaya, Colonia Real del Monte, International Central de Autobuses, Observatorio
Mexico Sur, Avenida Taxqueña 1320, Colonia Campestre, Churubusco
Terminal de Autobuses de Pasajeros de Oriente (TAPO), Calz. Ignacio Zaragoza 200, Colonia 10 de Mayo
If you need a printed copy of your tickets, you can get it in Office Max across the street (at the entrance of the building look left and you will see the logo).