I use an older style PASGT Kevlar helmet and suppliment it with one of those vietnam era flak jackets (USMC m1955(?)), mostly because I just had them laying around.

You can find the helmets for ~$50 and surplus flak jackets run about the same when you can find them. I know there are some german surplus flak jackets floating around the internet too. Obviously you'd need to check your local laws on the legality of owning body armor in your area.