There are two common currencies in Iran. Although the official currency is Rial (Rls), the currency which is used by people is Toman. Each Toman is equal to 10 Rials.
Rial is used in coins, banknotes and official arrangements. Rial is the printed currency but Toman is much more practical in daily life. So, when you need to pay for shopping, taxi or other stuff, you are confronting Tomans, not Rials. People will also abbreviate: for example, if someone tells you something is “4”, they mean 4,000 toman/40,000 rials. It’s confusing in the beginning, but you’ll get used to it.
The currency rate is different from day to day thanks to the sanctions. So it is advisable to compare the rates first and then buy Rials. There are a lot of online shops, so you can do the comparison easily.
To buy Iranian Rials, the easiest and fastest way is at an official money-exchange office, where the whole deal is done in seconds. Although you can do it at many banks in the cities, but it would take time and need more paperwork. Exchange shops (Sarrafi) can be found in most cities, usually signed in English. If not labeled “Exchange”, look for small shops similar to jewelry stores that have foreign bank notes in the window.
Don't forget that changing money in an exchange shop is much safer than doing so with a street moneychanger.