I know a number of people who do long term rentals in our region of Spain. While I hate to say anything good about facebook, most of the people I know monitor specific local groups on the lookout for people like you wanting rentals. They are usually also available on airbnb and similar sites but the property owners really, really like people who come to them direct!

So, choose where you want to go, see if there is a facebook group for that area and ask the question.