Hi,
Most of the on-line bike equipment stores carry light-weight body armor. Also, logon to mtbr.com and read the body armor reviews posted on this site. You don't need body armor designed for a professional downhiller and the reviews are a good place to start.
The mtbr.com site also has links to all the major vendors and on-line stores.
I don't personally wear body armor, although I have thought seriously about elbow guards after some particularily gnarly crashes.
Good luck!
Charley