Tourist easily can apply tourist visa in Kathmandu airport upon the arrival in Nepal. It may takes some time but you get nepal's visa in airport. We suggest that you should fill properly some proper  link/ dacuments before arriving in airport or you can fill in your country.
You should bring passport size photo and your orginal passport to abtain nepali visa in Tribhuwan airport in Ktathmandu.
You need to pay $ 25 USD for 15 days  tourist visa, $40 USD for the a month tourist visa and $100 USD for the 90 days  tourists visa.

See below Visa application form:

http://www.nepalembassyusa.org/resources/visa_application_form.pdf