looks like these two do:
Daily 10-hour tour to Taxco departing from Mexico City (wayak.mx)
Autobuses AERS Costaline | Compra tus boletos de autobús
also the first is a tour - but I am sure it is no problem ot take one there and then a day or two later take another back...
Also, consider the Red Arrow bus -
Destinos (flecharoja.com.mx)
you might also consider gong from Mex to Toluca and then catching a bus from Toluca to Taxco.