Hey ,
Booking online is the safest bet, and even if you plan to go to rishikesh and approach a tour agency which is also fine since there are many shops. You can always hop from one agency to another since they are all within 2 km vicinity, bargain to get the best deal.
Else taxi is always an option, but then you have to switch taxis , ( since tungnath temple is closed during this time of year so pilgrimage is nil ) therefore hard to get direct shared taxi .
regards
Arnab