You might be able to get some tips from Fabrizio at: firstname.lastname@example.org
. He seemed to know all of the tour guides in Italy--perhaps he knows some in Barcelona as well. You can also do some Internet searches. That's how I found one of our drivers. I researched companies and sent out an itinerary of what we wanted to do, and waited for the responses to come back.
We only had two days in Barcelona, and we purchased the two-day pass to the sightseeing bus and made good use of it. The only cons to this were 1) it didn't run late at night--we wound up taking a subway back one evening, and 2) you always had to follow the route, so if you wanted to go from point A to point F, you had to first go to B, C, D & E. Other than that, it was a wonderful way to see the city.
Good luck! Barcelona is an incredible city (what a mix of old and new architecture!)