Barcelona Jerseys

Since you will be in Barcelona I'm assuming you will be visiting the awesome team museum at Nou Camp stadium. The museum has the world's largest Nike Store where they sell everything and anything related to the team. If you want to cheat a little they can also be easily purchased in the US(probably cheaper too), at the Nike store at your nearest outlet mall.

or try soccer.com.

... FCB also has a couple of stores elsewhere in the city, one in one of the corners of Plaça Catalunya square. You could also get a cheaper one in one of the many souvenir shops, but you might end up getting a fake if you're not careful which shop you go to.

Many touristy shops sell them. in October we bought two Messi shirts in youth sizes. Good quality.
Twenty-five euro. Six year old slept in his until it was forcibly removed for washing.

We bought one for our grandson two years ago and he took the initiative to ask us to buy him another one when we are in Spain. He LOVED it, especially since it was brought from Spain just for him.
Donna

They also have a store in the airport I believe.

And in the train station.