You’d better have your money in Dollar or Euro because exchanging these two are easier in Iran and possible in each city. To exchange your currency to Iranian currency you have to pay attention that:

There are two prices for foreign currencies in Iran. The government price and the price of free market. For dollar the difference between these two prices at the moment July 2018) is about 90 dollars in every 100 dollars. Meaning that if you want to change your dollar to Toman based on the government price, you would waste 90 dollars by contrast to the free market. Before talking about the ways of changing money, you’d better know that our economy collapsed few months ago under sanctions and unfortunately there is no known place to change your money but do not worry about it. You can change your money in hotels, hostels, restaurants and it’s easy to find a person who suggests exchanging it for you. It’s strange but for sure you can travel in Iran in 70% cheaper.