I went to BA a couple years ago and purchased a pre-paid. You can't buy it at a convenience store though. You need to go to the mobile store and get it. It was a hassle more due to waiting for customer service than the form itself.

Shouldn't have any problems once you get it though.

I never found the pre-paid SIMs offered on the internet to be of good value compared to actually getting it in the country that I'm in.

You should google for Argentina or Buenos Aires Mobile or Movile and find the local websites. I recall some of the carriers have English as an option. I think Telefonica was one carrier I recall.

Good luck!