It’s been a while now, but bus schedules and tickets for Valladolid are now available online at www.ticketbus.com.mx, along with all the other ADO and associated lines.
The one exception is service by Autobuses Centro, which runs to Izamal–this is an unaffiliated second-class company, and it runs from a terminal on Calle 54 at Calle 37. (The sign on this terminal actually says ‘Autobuses de Oriente,’ but all that service is now at the central bus terminal, with ADO.)
ADO’s deluxe buses, referred to as UNO in the guide, are now called ADO Platino.
In Piste, next to Chichen Itza, the two bus stations have changed their roles a bit. The one on the east end (closest to the ruins) is where _all_ buses stop (first- and second-class), and the one on the west end, near the main intersection, is only for second-class buses. But you can buy tickets for any service at both stations.