It depends but if you stroll arena Plaza de Catalunya and Passeig de Gracia it's not unusual to find them, but for the highest chances of having some fun I'd go to the marina in Barceloneta and the various clubs/bars there... Opium or Pacha to mention a few, but basically point the clubs by the sea front between Barceloneta and Port Olympic I'd say. I hope it helps.
Just got back from BCN.
We went to Opium both nights. Pacha wanted crazy money for table service and that was the only entry option.
I wrote about this more in the EAS thread, but we invited 3 random Spanish girls back to our hotel suite for an afterparty. Things got wild and one of them turned out to be a TS. Now I've been seeing TS escorts since 2008 and I genuinely had no idea until she got her cock out.
Admittedly I was drunk and high on coke and didn't really look at her closely, but absolute lols.