Andrew -- About the best way to find those is to look for a firetruck, tow truck or some other 2-ton with a body on it using the long running boards and fenders. You will most likely have to buy the whole truck to get them.
Not to say you can't find them if you look hard enough. I'm just saying that I've never seen them except on a few trucks with the bodies I've described.
As Bill says, you're best bet is to try and fab them yourself. Contact Jim Carter (Jim Carter Antique Truck Parts -- look in our links section) -- he has a HD '46 1-ton with those fenders and running boards. He can probably send you some measurements and photos if you end up fabbing your own.
Here are a few pictures of it (Plus a '38) John
