We have rented an apartment in Barcelona about 8 years ago through an online site, but, sadly I cant find any record of which one that was. There are probably far more around now anyway. For a reliable company try one recommended through Barcelona.com. If you haven't visited Barcelona before, I wouldn't be too worried what area you are in, it is not one of the hugest cities in the world, and is quite easy to get around. If I remember rightly the Barrio Gotica area is not huge and extremely central. You would actually want to get out of that area after a day or so anyway. The underground system in Barcelona is quick, cheap,efficient and easy to use. Getting to Sagrada familia was the longest walk we did, and we walked a lot, and we were based way down towards the sea. It is a great city and lots of fun. Beware of pickpockets near las Ramblas.