If it's what I think it is, it's just a galvanized sheetmetal gate that fits in clips. I doubt you could buy one from parts dept on a unit that old, but you could probably make one with some tin snips and a 1/2-hour. If you open the access cover on the outer skin of the WH, and look in, you'll probably see some slots around an opening towards the burner. You need a piece of metal to slide into those slots and cover the hole to the burner; nothing fancy or exotic. Leave a tab on the top and bend it out so you can grab it if you ever want to remove it easily.
I'd be more worried it is 14-years old and could go any time now.