I suppose you could carry around a level III ceramic plate in a backpack, which is designed to take a water pouch but at 6lb weight it would soon become a little tiresome after a while.

http://www.vestguard.co.uk/ballistic-plates/ceramic-level-iii-ballistic-plates.htm

You would also probably need a ballistic helmet as well such as a British Mk6a or Mk7 (Mk6a's can be had for about £40-60)

Who do you expect to be shot at from?