Hey BHB,
Have you tried looking for English Walnut hulls? It is possible if you can get a european nut hull, you might be able to find it. In this day and age, the transportation of certain plant material can be difficult if not impossible, all very understandable. In my recollection, the english walnut has a similar hull to the black walnut and starts green switching to black. If there is a colour difference, I am guessing it would be subtle.