Coming from Tortosa to Amposta on the C12, go straight over the roundabout (3rd exit, signpost Amposta sud, Tarragona/Castello).  About halfway along that straight stretch, turn left on a small road by a tall Leylandii tree opposite Suimplast Materials  The warehouse is about 200m on the right called Fustes Roca.  It's only small but has most of what you'd need at good prices.  They'll cut for you also.  No English spoken but just look around and list the lengths/sizes you want.