If you have a friend that knows mountain biking well, then going used is a good approach. Pinkbike.com is a good place to start for people in the USA/Canada. RE

If you have a friend that knows mountain biking well, then going used is a good approach. Pinkbike.com is a good place to start for people in the USA/Canada. REI is also a good place for reasonably priced, new bikes.