It is very hard to find. Valentine Inc. carries the 1/3" which is very close in size - only 0.042" smaller.
You'd have to order a whole roll of it but I'm sure you could sell the rest of it. It is very popular among mouse breeders as both the 1/3" and 3/8" sizes allow for the mice to eat food resting on the wire but hoppers are unable to escape through it.
I assume that is what you want it for.
Some of Valentine's pages are down right now, but if you go to the link below and scroll down to hardware cloth you should be able to find the 1/3" product.
http://valentineinc.com/WIREMESHLINKS.html