You can ask for unlocking around at pretty much any legit phone shop, or the little shops in the galeria. So long as its not a new model it should be easy to unlock, then you can get a pay as you go SIM. Movistar probably the best of a bad bunch for a network.
Also worth checking for unlock codes online, there are sites about that give you a code to type in, which unlocks your phone instantly. Failing that you can get a brand new phone for 100 pesos.