You can get official Barça from the FCBarcelona online store
You can also get them from the FCBotiga Megastore at the Camp Nou from Monday to Saturday from 10.00 to 19.00. On Sundays without a match at the Camp Nou, from 10.30 to 14.30. On match days at the Camp Nou, the FCBotiga is open until kickoff.
In summer, on workdays the store is open until 20.30, while the FCBotiga Megastore is closed on December 25 and January 1 and 6.
Other stores where you can purchase official products are: