If you have an old phone hanging around at home and it is 'unlocked to any network' then there are lots of pay as u go sims to choose from and buy in Spain.
If your old phone is not 'unlocked' you can get this done for £5 - £10 in most independent mobile phone/accessory shops.
Or you can buy a phone for £10-£20 from carphone warehouse, cos theirs are all unlocked.
I bought a happy mobil sim in Dos Mares shopping centre (u dont need to go that far), no charge really cos I paid 20 euro but came with 20 euro credit.
Have to give NIE number, and is easy to top up at most cash machines.
Great because if I'm not there using, then I'm not paying, and No big bills when I get home from T-Mobile anymore.