Good search engines for flights.....
US - India
East coast travelers- JFK/NY- Delhi is cheapest that I found.
West Coasters- LA-Mumbai seems to be a good bet.
Once in India:
Fly to BAGDOGRA AIRPORT.
The Sikkim tourism department will take care of you from there with a shuttle to Gantok.
I'm looking into the incountry flights right now. And waiting to see if there is a chance for discounts for international travelers.