Check at any welding or fabrication shop.
I get mine from the scrap bin at my local welding shop.
I have tried calling up some of the steel yards like logan steel, who is a big one out here, and no luck. they dont even have any idea where to get it from.
I will be back on the phone this afternoon! Oh, and Minisimp, great post on building your square d-shaft! that is what i am using as my refrence!!!
Cheers for that!