Welcome to MR, andilubio
I live across the Atlantic and don't know any bookstores in Spain. You could check our Wiki for bookstore listings.
Possibly a member from your area will offer some advice when they see this post.
If your mom likes to read public domain books, feedbooks has a section in Spanish.
The Wiki also has a E-book Reader Matrix
with more information about the various devices.
I'm sorry I can't help a bit more, but I have the Sony 505 and only know about purchasing books in English on this side of the Atlantic. For me, the Sony 505 was pretty easy to use.
Maybe search through some of our forums for more information. (What a good son you are for helping your mom get a Reader