You should buy your tickets beforehand. You'll have to have them in your hands before you go to the event. In other words, they don't send you a confirmation with a bar code that you present to someone at the event.
One option is to find someone who understands Spanish to help you buy the tickets. If you're going to be here a few days ahead of time, you can go pick up the tickets at an office on Alsina in Microcentro. You can also have them sent to you in your home country. This will cost an additional $200 Argentine Pesos.
I could also buy them on your behalf if you Paypal'd me the money. I will be buying them for myself soon, so I could just as easily buy yours and pick them up. I could hold them for you until you arrived in BA, or send them to you via Federal Express or DHL or whatever service you'd want. If you'd like to go this route, send me a message, and I'd have no problem helping you out. Someone correct me if I'm wrong, but if I issue him an invoice through PayPal, the purchase would be insured on his end, no?