It seems here that wood is sold by specialists. We hunted around a lot of builders merchants, many of whom had odd bits, until we were told to go to a specialist.
We now use 'Fustes Roca' in Amposta. There is a little sign on the new road in from the N340 at the southern end, second exit south of the Ebro.
Plywood is not cheap. Budget €50 for a 2500 x 1200 x 18mm sheet of exterior ply. (Yes 2500mm long)
Try these...
http://www.paginasamarillas.es/search/madera/all-ma/tarragona/all-is/tortosa/all-ba/all-pu/all-nc/1?cu=madera+tortosa&ub=false