In that price range you have two options. 1. Craigslist (good deals to be had, but can be spotty.) 2. If you want new your only option is really
www.bikesdirect.com
At that site you pay for the good components and not the name on the frame, so buy as much as you can afford. Motobecane and Windsor are actually pretty good frames, they just aren't names synonymous with mountain biking.
In both instances you'll need to have a size in mind... for this I'd go "look" at a bike shop and have them recommend a size for you and then take that info to the web.
Good luck.