I can't tell you for Barcelona specifically, but I find the best thing is to look for promotions, rather than specific hotels.

First, I figure out what areas I am willing to stay in. I also have a vague idea of how good the hotel has to be (but this is more a know it when I see it sort of thing).

Then I look at a site like www.hotelscombined.com . With this site, you can see numerous hotels, and you can see the prices charged by various websites for the same hotel. Sort by price (I do anyway), and keep looking down the list until you find a hotel that meets the criteria you have in mind. Then I look it up on TripAdvisor and various sources. www.kayak.com is another good site for this.

Often the hotel you had in mind from the start is on one of these third-party websites, but at a cheaper price than you can find on its own site.