I am traveling from Barcelona to San Sabastian in early October and need information on how to purchase tickets for the train. Also, how long is the train ride?

The train company's website is:
www.renfe.es
You can purchase tickets online.

I went to the Renfe office in Madrid to buy train tickets. I had the impression that you could buy any tickets at the office. It's located right behind the Banco de Espana. I didn't look for an office in Barcelona, but there might very well be one.
Hope this helps.
Cristi