Banks and licensed money-changer booths (marked obmen valyot) exchange US dollars and other major currencies. Trying to get change for a 5000som note will likely be met with a look of horror, even in cities.
There is no black market for currency transactions and changing money back out of som is not problematic. We quote prices in the currency that the businesses themselves use. That's normally som, but can be US dollars or euros for some hotels and tour companies.
If you need to wire money, MoneyGram has services at main post offices and Western Union works through many banks.
ATMs are increasingly common in all major towns. Many dispense both US dollars and som and work with Visa, but for Mastercard and Maestro look primarily for Demir banks across the country.
Restaurants and Bars Tips of 10-20% will be included in bills when appropriate.