Have a great time Barbara!!
I did a search for you for fabric stores, but did turn up anything. If you go around, have this written on a piece of paper: Donde encuentro una tienda de telas? (where can I find a fabric store?) and maybe the hotel people can lead you. If you do find something, be very open minded about fabrics other that cotton, since most likely that's all you will find...no need for quilts there.
And (I will sound like a mother now) don't venture too far from the tourists area. The food might be better and cheaper, but stay safe.
Have fun at the beach!!
Maria