Try amazon.com - USA
They have a range of Spanish language books at OK prices. Most will ship with postage at good rates too?
Umm… the selection seems a bit limited. Will post again if I find another site…
PS - Are you near a large city?
I have found a few good second hand books stores on my travels….
Amazon Uk will ship some items for free (you need to make a min order of 25GBP)
Hope you find what you are looking for
PS- In Fuengirola there is a free Library (multi lingual) at the Lux Mundi centre. Lots of Spanish classics there I am informed.