I wound up getting my bike from Amazon as there was no place within a reasonable distance to me selling it. As of now, it is no longer available there. My wife’s bike is available online depending on your location (they are based out of California and don’t ship all over the US – check GT’s website for details), but I preferred getting it from my local bike shop, and as it turns out, I was able to negotiate the price down $50 from retail and get the complimentary tuneups on both our bikes, which is a great perk. Consider buying from a shop if possible for that reason alone.
What dex says is true, Craigslist can often be the best option. It’s how my brother, a seasoned biker out west, tends to acquire the best deals. However, being new to the sport and not knowing anyone really into biking around my area, I decided to buy new as it felt like a safer option. I think Diamondback is kind of known for having pretty good bang for your buck on entry level bikes, though there are plenty of other companies, both big and small, that have good options. You just have to do your research and figure out what you want. Jeff provided you with a good link to a few of the options. This is another page I visited at the start of my search: http://www.davescheapbikes.com/cheap-mountain-bikes/
It should help you to get started.