You will be able to get Barcelona tickets for any home game (bar the Real Madrid match) on the day or days leading up to the match at the stadium. You can also buy online in the few days leading up to the match.
Remember that the day of the match (Saturday or Sunday) and time (can be as early as 7pm, as late as 10pm) is only confirmed a week or 10 days before the match - this is to accomodate TV - so plan your trip accordingly.