I got my tourist visa upon arrival in the Kathmandu airport. I accidentally did not bring enough cash with me, which made the process a lot more difficult. Remember all the necessary materials! If you want to make the process even more convenient you can also mail in the visa application to a Nepal Embassy before leaving and have it made. Although it was not very difficult to get the visa upon arrival, it would make it even quicker to get through the airport.
Here is what you need upon arrival to get a visa successfully:
-A passport photo: I would bring at least four passport photos before arriving in Kathmandu because you seem to need a passport photo for everything here. Including trekking passes and passes to get in to Bhaktapur. So although you only need one for your visa it is good to bring some extras.
-Cash: 15 day tourist visa is 25 USD, 30 day tourist visa is 40 USD and 90 day visa is 100 USD: It can also be paid in euros, but not in rupees. The nearest ATM is outside the airport so it was really hard to find and the airport is so hectic that it made it a way longer journey than it should have been. Bring cash with you!
-Visa Application Form: Print and fill out this form before landing in Kathmandu. Here is a link to the pdf: http://www.nepalembassyusa.org/resources/visa_application_form.pdf
-Passport: of course!
-Immigration form: On the airplane they will hand out an immigration form and it is necessary to fill out the visitors section as part of your visa application.
If you have all of these materials you should be all set! Have a great trip.
For more information visit the iTrekNepal website