I purchased some of the shanks at a tractor place that sells new and used tractors of many brands. They sell lot of implements also. I was looking at their grapple rakes, and they seem high with the price of steel. As I walked by the box blades, I took one look at the shanks and came up with the idea of my grapple/root rake. These shanks are 3/4 thick and very hard steel. I purchased some of them at the tractor place, and some of them at TSC. The ones I got from the tractor place were three hole shanks at $15.00 each. The top shanks have four holes and are a little longer. TSC had them for $29.00. so I asked them if they would match the price of the other tractor place, and they did.
Anybody that sells box blades, should have them in stock, or can order them. One of the tractor places wanted $50.00 for one, and I just thanked him and kept on looking
Another option, I drew up a design of a grapple fork on one of my Cad progams, and had a steel fabricator cut it out on their cnc machine. A single 1/2 inch shank cost me $30.00.
I would try and get the shanks locally, because the shipping would be rather steep.