A lot of people use doyouspain.com to find car hire. You will find on airport and off airport hire companies listed. Check their individual terms and conditions of hire such as excess, fuel and deposit. You can buy an annual excess cover in the UK for about £40 which is much cheaper than the insurance feared by the companies themselves but having this does not mean you don't have to leave a deposit, it just means you have a policy to cover any loss of deposit due to damage, etc.
also take a look at laracars.com. Not used them myself but they claim to have fixed prices and no hidden costs.