There is always a lot of activity on Las Ramblas - in many ways it's the center of the city in Barcelona.  As you get closer to the waterfront you will notice more women who might be "available", but as always, you should be careful.  Don't get caught buying the $100+ bottle of champagne in their "special" bar.  One block off Las Ramblas you can also fine more women.  I don't have any experience with escorts in Barcelona and can't recommend anything along those lines one way or the other.  For what it's worth, I found Barcelona a very easy town to get laid in for free.  As always, speaking the local language is a plus.

Good Luck!