The buses to Ayacucho leave from the nearby transit hub of San Clemente (easily reached by a short taxi ride from Pisco). The taxi driver was very helpful and showed us the two companies that service Ayacucho: Molina and Anhanza. Both of these companies offer only basic service for the 8 hour trek through the mountains. The buses leave at 12:00pm, 12:30pm, and 7:00pm. We got there a couple of hours early but Molina was all sold out, leaving us with Anhanza (35PEN/CAD$12.50).
We grabbed a coffee from a small restaurant near the bus stop and waited for our bus to arrive.