You might try going to some of the big chinese stores that sell everything, there are 2 big ones near the big Carrefores shopping centre near Pamones. Take the first exit after 2nd Los Cortijillos exit coming from Jerez. Cross the motorway and go past Carrefores, they are on the service road that runs parallel to the main motorway just past the Mercedes dealership.
Also the best place to get fabric at really good prices is in the street markets. La Linea has one on Wed mornings and they have loads of fabrics at really good prices and much of it is 280cm wide. Mostly its curtain fabrics but you can sometimes find other types. There is another market in San Roque on Sunday mornings and you will find a large stall there selling all kinds of notions.
The last suggestion is Princess Silks in Gibraltar on Main Street accross from Marks and Spencer. They have lots of fabrics and notions. The fabrics are ok ...not many natural fibres and the prices are not great but they do have some beads and other crafting supplies.