Yes it is possible to book a ticket online for example on orbitz originating from KTM. But I strongly suggest you not to because the rates will be very high if you issue a ticket from a country other than where it is originating from.

I recommend you use a Nepali travel agency in Nepal. A few America-based Nepali travel agencies have full service travel agencies in Kathmandu, you should get a better rate from them than most other agencies.

Look at all the ads in this website and see if any of them do tickets from KTM. Also, an open ticket is possible, but will cost you a little more, maybe you would be better off buying a confirmed date ticket and then change later. Ask your travel agent for details.

Hope this helps.