There are several buses a day from terminal Tasquena in Mex City. The ride takes about three hours. The Pluss buses are super comfortable; they even have a pillow for the feet. The ride there and back takes about US$20-25.
There are plenty of buses headed to Taxco. I took one from one of the southern bus stations in Mexico City. There are several other buses to direct locations within Mexico. If you are low on time, then head to Mexico City where you'll find more services.