There are quite a few sources. The only issue, of course, is almost none of it is in English so there's an intertwine of cultural as well as language barriers to the bigger, Western world over and above the foreignity of their wrestling style. For instance, Matt already mentioned that we're working on Panama via Segunda Caida and, from what I can tell, we're moving on uncharted territory pretty much. It's a little bit daunting, frankly, but latin America is filled with a kaleidoscope of rich history when it comes to wrestling. It's my hope that I can, little by little, help to unearth it to the rest of us.
El Salvador, like Guatemala, has quite a lot of its history already mapped out by a few historians. I'd recommend starting here and here for Guatemala. As for El Salvador, there's a great summary of its history by Manuel Canadas (part 1 and part 2). The same website has a few other articles that might interest you. Here's another.
Salvadorian lucha looks nuts. Here's Tony Jackson jumping from the third rope to perform his signature "tope mortal" on el Bucanero: