Note: At the time of writing June 2022 it is very common to buy insurance when booking a hire car on internet. When you arrive to pick up the car people are often told that the insurance that you have paid for while booking the ticket on internet is not valid and that it is necessary to purchase more insurance. This scam is very common.
It is no cheaper to rent a car when you are in Spain than booking it on the internet before you arrive. The best thing to do is to visit several car hire websites and compare prices. It is a good idea to use multiple comparitive searchers to get the best deal.
With the bigger companies you can pick up a car from one place and drop it off in another.
Rental Car Multiple searcher - Click here to search all the big car hire companies that operate in Spain