I have found Spain is rife with stones and sacred pagan sites before the Roman Catholic Army thrashed everything in its onslaught. Mind you, they are hard to find unless they are really important and here is a general book that includes the main ones along with some other Roman and Medieval paraphernalia:
Spain 1001 sights, an archaeological and historical sites
James M Anderson
Which area are you going to? There are some regions which are very very dense, with thousands of them in an area the size of Greater London.