Hmm, interesting, I would have thougth 16's would have had a stronger following around teh world as Land Rovers Defenders have been using 16's for a while now. That is a big consideration though. One of the reasons for going with 33's is that they are still more of them out there then 35's and up. At least when you exclude trailer queens. Not to mention, Expedition vehicles tend to be truly 50/50.
In Panama and Costa Rica, you can still get both 33 and 35's in the capital cities (San Jose and Panama), but outside of that, you can only get 33's (and not in all the small towns). If you truly want to go with a tire available
everywhere (including very small towns), you would need to drop to a 31" (and if it is Latin America you are talking about, a 265/70).
As for 15 or 16 size, the new Toyota HiLux Pickups all come with 16" wheels, so getting an R16 tire should not be all that difficult (as long as you dont mind going to metric, it should be available in most major towns). I know the HiLux comes in either 265/70 (also the stock setup for the Fortuner, which is also popular around here) or 205 from the factory depending on what you buy, and since that is the best selling pickup in both countries, I am sure you would find tires for it almost anywhere...
In fact, even the Landcruiser 70, which is still popular in Panama comes with 16 inch wheels, so at least down here, R16 would not be that much of a problem (also, Costa Rica sells almost as many Land Rovers as all the rest of Latin America combined, so with so many Defenders here, R16 again would be readily available).
Anyway, just input from my side of the world, should you ever head down this way to explore (and if you do, make sure to give me a heads up!).
Felipe