If you just want one ticket, you shouldn't have a problem. Main problem is finding two or more seats together.
You can buy tickets for the Leverkusen game now from (for example)
http://www.servicaixa.com (look at the top-right of the page for a choice of languages). When you arrive in Barcelona you just collect your tickets from any ServiCaixa ATM (you need the credit/debit card used to buy the tickets).
Very few games are a sell-out, with more tickets becoming available in the days leading up to the game (as season ticket holders release their seats back to the club).