If you are traveling from a U.S. border city, into Mexico, check with Greyhound . At major border crossing points, they usually have bus connections, with the main Mexican terminal, across the border, for a few dollars. Another, expensive choice is to take a border taxi to the Mexican bus terminal. Sometimes, that's your only choice. Try to think of the country of Mexico, as if it were a very large town.
Every road has at least one bus.
I don't care where you are, in a city, on a main road, a small road, a side road, or aliens dropped you off in the middle of nowhere. Just as in your home town, if you wait a couple of minutes, a bus will come by. If you want to catch it, just wave your hat in the air. In Mexico, there could be more than one bus terminal, and several bus companies per town. Even small towns may have a couple of bus terminals hidden in store fronts, on opposite sides of the same street.