If you have an unlocked GSM phone, Im told that the best option for a prepaid SIM card is CTI, not Telefonica or the other GSM operators. One useful trick is to ask for a prepaid SIM, and when you are given the hard sell for the whatever plan or whatever months just tell them "turist" and that will cut the sales pitch. If they have a simple SIM card they will sell, else you can save time and go elsewhere.