Try amazon.com - USA
They have a range of Spanish language books at OK prices. Most will ship with postage at good rates too?
http://www.amazon.com/Books/b?ie=UTF8&node=301731
Umm… the selection seems a bit limited. Will post again if I find another site…
PS - Are you near a large city?
Malaga/Madrid?
I have found a few good second hand books stores on my travels….
Try this…
http://www.amazon.co.uk/gp/search/ref=sr_nr_n_2?rh=n:266239,k:books+in+spanish+language,p_n_binding_browse-bin:492564011,n:!1025612,n:62&bbn=1025612&keywords=books+in+spanish+language&ie=UTF8&qid=1312793603&rnid=1025612
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.