Tribhuvan International Airport under the Department of Immigration has the authority to provide Visa on arrival to tourists flying to Nepal. You can also consider getting Visa from Nepalese Diplomatic Missions stationed abroad prior to your arrival. If you have obtained a visa from Nepalese Diplomatic Missions, then you must enter Nepal within six months from the visa issued date. The total stay starts from your arrival in Nepal. Visa obtained on arrival at the entry points are the tourist visa. If you wish to change your visa category then an application along with the required documents are to be submitted to the department of immigration. To get a visa on arrival, certain fees must be paid, which are On Arrival Visa Fee and vary as per the length of the visas as: 15 Days visa costs 25 USD, 30 Days visa costs 40 USD, 90 Days visa costs 100 USD. Further information can be referred at the official website of the Immigration Department at http://www.nepalimmigration.gov.np/page/tourist-visa.