It is almost impossible to locate veal knuckles, or those of any four legged beast, unless you make orders

for a restaurant and can order a case, or have a farmer who will part with them. Knuckle bones impart a richness to stocks largely due to the
marrow in the bones. The knuckle is a hinge joint.
I think you could successfully substitute osso buco, chopped oxtail, or any bony meat you can find, like veal shanks. Although specialty stores might carry such bony meat, I find neighborhood grocery stores with a less affluent clientele are inclined to stock them more often, and a wider, if erratic , array.