In most European countries you don't 'barter' in bricks and mortar shops, but you can barter when you buy something at a market stall.
Generally, European quilt shops offer the same fabrics that you get in the US. They are just a lot more expensive (often double the price of what you pay in the US).
I always enjoy looking for textiles which are typical for the country/area, and that you can't easily get abroad. You can get some lovely lace in some areas of Spain. I also always look what second-hand shops have to offer when I'm in other countries. I picked up a very attractive and heavily embroidered vintage linen tablecloth in Switzerland that I'll cut up and use in a quilt one of these days.
Lorchen
From the edge of Sherwood Forest, home of Robin Hood
Good question. She may be able to barter more in the market places. I will say that many of the fabric places have more home decoration fabrics than quilting cotton. But there are shops around..one in Girona Spain..it is probably 70 miles or so from Barcelona.
JL Quilts in Madrid http://www.jlmarquez.com/
In Spain the metric system is used. She would buy fabric by the meter (1 yard (yarda) = 0.9144 meters (metros). If she buys by the meter...then you will receive 1.09 yards.