Som (S), sometimes spelled s'om or soum is the currency of Uzbekistan. The highest Uzbek note is 50,000S that is recently introduced in 2017. US dollars are easy to exchange into SOM in and you need to carry a lot of cash as most of the transactions are in cash.
You can change your currency officially at the airports, hotels, national bank of Uzbek, as well in several private Banks. ATMs can be found in Tashkent and outside of a few hotels in the capital. Avoid using ATMs on Sunday as you will mostly found them out of cash.
While buying souvenirs make sure to bargain. You can avail some good discounts if you can properly do bargaining.