You can buy your sim card in El Corte Inglés or wherever cell phone shop you want (vodafone, movistar, orange or yoigo). I'd recommend you to go to The Phone House shop where you can buy a sim card from one of these companies and perhaps you can get some good offers. Movistar is the company with a wider and a better coverage (cobertura) all over Spain, in case you go to the mountains or the countryside. But in cities, all cell companies work well. My cell phone is from an internet provider (www.simyo.es
), and uses the Orange network. Un saludo.