If you're coming only for a week, then it's pretty pointless buying one in Spain. If it's internet you want, you might just as well use Wifi networks, there's enough of them. Just go to a bar and ask "¿Tienes Wifi" (pronounced - tee-in-ace wee-fee?), and if you need to ask for the password, say "¿Cual es la clave del Wifi? (Pronounced - Kual es la cla-be del wee-fee?).
If you've got any more questions, don't hesitate in sending me a PM. I've lived here for half my life now, and I'd be happy to advice you on any help you may require! =3