Tickets for new season not on sale to general public till tomorrow. They go on sale today for members only.
Sevilla won't be a sell out, but the best seat will already be gone.
Best to buy as soon as possible to get best available seats, or by the time you arrive, you'll be sat in the clouds.
Best place besides the official website,http://www.fcbarcelona.com/football/first-team/calendar to buy tickets is ticketmaster: http://www.ticketmaster.es/nav/landings/es/deportes/entradas_fcbarcelona/index.html where you can collect your ticket on arrival at any Servi Caixa bank ATM in the city, using the payment card you paid tickets with.
Any other questions, please ask away.