Are there good websites or other sources to find apartments/homes for long-term rentals or purchase in Madrid, Seville, Barcelona, other major cities and charming villages nearby those larger cities?
I would try http://www.idealista.com/en/ or http://www.fotocasa.es/en/. They are the two biggest portals in Spain, and probably have the largest selection of properties listed in the places you mention. However, I have no idea what the agents that list with them are like. It’s a bit hit and miss I’m afraid.