Montserrat Museum

It seems that there is an entrance fee for the Montserrat Museum. Can I purchase it at FGC Placa Espanya when I purchase the round-trip train tickets? Or should I buy it at the Information Office on the site of Montserrat? Either way, how possible will the entry quota be sold out? If this is possible for a late Aug trip, should I get it from the Montserrat official site instead?