The Turkmen manat (TMT) is the national currency. Besides dollars and euros, Russian rubles, Kazakhstani tenge and Uzbekistani soms are in demand. Ashgabat has the main offers, with more in Türkmenabat, Daşoguz, Mary and Balkanabat.
Officially, currency in Turkmenistan is exchanged at banks, so it is worth checking the conditions and limits before exchanging. The map shows which points and offers exist in your city.