If you mean the Bus Turista, the hop on hop off one, then you can buy a ticket on the bus.

luci

There is a booth in the Placa D Catelunya at the top of the Ra,blas. If you go into Barca on the train you get off at the station under the Placa and the booth is just across the road on the island. 2 years ago the fare was 17 euros each per adult and there were 3 routes. You can hop on and off and the routes intersect so you can change.