Disclaimer: this site is meant to promote the building (not selling) of Halo related armor and props. Take the time and I think you'll find that you actually do have the ability to make some really amazing stuff!
That being said, if you really want to buy stuff, there are some options. As was mentioned before, if you stick around long enough and contribute enough you'll be granted "Infantryman" status. This will allow you to view the Classifieds page. Another option is the aforementioned eBay, though it's pretty rare to find actual armor there since mostly people are selling replica guns, for some reason. A third option is
http://www.westerfieldstudios.com/. Expensive, and I cannot attest to the quality since I've never purchased anything from them, but you might want to give it a whirl. Good luck!
P.S. I would definitely recommend making your own. You'll love it. Seriously.