Hi, Justine,
There's only one bus station, and all companies leave from there. The address to plug into google maps is:
Plaza de Camilo Diaz Baliño, 0, 15704 Santiago de Compostela (Estación de Autobuses de Santiago)
I usually stay somewhere near the start of the Rua de San Roque, and it's an easy 15-20 minute walk from there.
Buen camino, Laurie