I bought an unlocked dongle of ebay and a pay as you go Spanish vodaphone  3g data sim card of ebay too. The Sim card was £12.95 with 10 Euros credit and free postage from Spain.

The guy selling the sim cards has a website: www-simcardspain-es
That is his username on ebay too. I've had no problems with this. He has some on ebay at the moment - Item number: 220901043061


There is also a Vodaphone shop on the main drag in Tortosa where you can top your card up. Or you can do it online. If you try to buy a sim in Spain, be prepared for the usual Beurocratic crap needing passports, Nie etc.

I bought mine from this guy and avoided all of it.

Hope this helps.
john