I do not know of any blue prints for sale, but look up some commercial units and down load there manuals and there parts diagrams should show all the parts and how they interact, to each other,
do a paten search and see if any are on the patens there just about a set of blue prints with out the measurements,
basically the spade is a triangular shovel with a few light bends in it for strength with a guide that attaches to the top end and a hydraulic cylinder to push it into the ground, a reasonable clear picture here of a light duty one, Florida Shores Truck Center: HYDRAULIC TREE SPADE FOR SKID STEER ST256