I have a 92 Toyota Previa van and wonder if parts are available if/when I might need them there. Any/all? parts of the country?
I'm sure that you'll be able to find parts in most places. I see a lot of Toyota's in Guatemala, it seems to be a popular make there.
Toyota is the most popular car make here. It would be extremely unlucky that you wouldn't find any car part here, unless it is a model that was only manufactured for the U.S. market.

Agree with the above but you should know that mechanics normally have NO parts on hand. They have to send someone to buy parts for any car and that can sometime take a day...just to get the parts, so be aware of that and be prepared (books to read, snacks to eat etc., etc.)to spend an unplanned overnight somewhere along the way. Look at it as an adventure.