niten ones are as light as you can buy.
bokkenshop do them as well,
though i think if you are buying the set it works out a bit cheaper at nine-circles.
because of my shoulder problems, and being a wee girly,
a kind member in my iai club made me one from birds eye maple
no good for contact work but extremely light.
if you make your own you can do it to suit your length and thickness requirement also
hope the wrists get betterer