Any suggestions for best way to buy temporary cell phone in Spain or sim card for use in Spain?
There is really very little to suggest. Go to a store, pick a model of phone, pay for it and start using it. If you go to an official store run by one of the three major operators, your phone will be "locked" (can only be used with a SIM from that operator). If, on the other hand, you go to one of the "fone bazars" which abound in any city, you can, for virtually the same price, purchase an "open" phone, which you could later use with a SIM card anyplace in the world that has similar communication (e.g. GPRS, 3G, etc.)