It is advisable for travellers to purchase a SIM card upon arrival in Bhutan
We got an E-Sim from our tour operator when we arrived in Bhutan for first time,
personally I think this is the easiest way to have internet access in the entire country.
Otherwise you can always purchase your tourist SIM from the airport, mobile operator office or from any authorised dealer in larger towns like Paro and Thimphu.
Most of the hotels offer free wifi. Internet connection is generally good in bigger towns like Thimphu, Paro, Trongsa and Bumthang. Some remote valleys may have intermittent connectivity.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.