I'm travelling to Mexico later this year and wondering if anyone has had experience with SIM cards from companies like Telcel or Movistar. I have an old phone that I'll use (hopefully I'll be able to unlock it) so that my current phone doesn't have issues with iMessage. It's an AT&T Avail 2 (also called ZTE Z992) which I have the unlock instructions for. I bought it in the US two years ago.

What's the easiest and cheapest way to get a SIM card and load it with an American credit card?