Welcome to the forum! I'm curious as to what exactly got you interested in trying the rawpalaeodiet? It's just that a more common route is that people become very ill, try most diets out there, fail miserably with them or only experience a few benefits therefrom, and then come up with this diet as a sort of last resort which really works.
We don't seem to have any active members from Spain here, unfortunately. From my own experience, I can suggest the following, some of which might work:-
a) Look around for any local farmer's markets. In the UK, farmers' markets are sometimes amazing with incredibly cheap raw animal food of grassfed or wild quality. Also, fishmongers are an option since Barcelona is on the coast. On the other hand, my own experience is that buying fish from the over-fished Mediterranean is often expensive. Still, there should be cheap varieties here and there, and more to the point, they will be wildcaught fish as long as you don't buy farmed fish like salmon.
Here's some googled websites which might be of interest:-
http://www.barcelona-metropolitan.com/eating-and-drinking/good-food-guide%3A-organic-produce/http://paleohacks.com/questions/4839/grassfed-meat-in-spain#axzz2YpoRSFZPb) Check out an online organic food directory to find which organic food stores are nearest you. In the UK and the US, there are several such sites. Obviously, you would have to know Spanish for that.
c) Look for a wholesalers, probably at the edge of town, which sells wild game. You would get it a lot cheaper than from a wild game butcher.
If you have no problems with raw dairy(many do) , you could look for raw dairy sources in Barcelona:-
http://www.realmilk.com/real-milk-finder/other-countries/#spainOnce you've gotten into this diet a year or two, I would most appreciate it if you could post on rawpaleoforum some details of your food sources. You don't have to be specific, in case you are worried that your few sources get swamped by rawpalaeos, but a general idea of the dos and don'ts would save people a lot of time. Thanks in advance.