This question is almost the flip side of Is it possible to get Mongolian currency in Sydney?
Is it usually practical (though perhaps not always optimal, related questions: Should I change most of my money in my home country or in the destination country? and When traveling to a country with a different currency, how should you take your money?) to transfer US dollars into the local currency either while entering, or after entering, the country? Assume you're flying by plane into a major airport for the country, rather than doing a land crossing into the middle (or edge) of nowhere.
I wondered whether some countries would oppose the US dollar for political reasons, but even North Korea and Iran allow you to either use or exchange US dollars. (Are there any countries which outright ban US dollars?)
I assume the main provisos are that you should have some local currency as soon as you're in the country to buy accommodation, food and transportation in most but not all countries, and that some methods of money exchange can have very long queues and only operate during business hours.