There are several companies that offer pay as you go cell phone plans. The biggest names are Movistar, Personal, and Claro. I have a Movistar phone that I bought a year ago and it still works great. Just walk along any major street in BA and you're sure to find a place, but if you want a specific location, I know that there's a huge Movistar store near Florida y Corrientes (possibly on Maipu) in el centro. Or just ask anyone in a locutorio, or kiosko for the nearest place. You buy a phone, have them activate it, and after that you buy cards (at any kiosko) to recharge your account. Remember that text messages are way cheaper than calling. Hope this helps.