Look for veal shank. The main thing for your stock is to get bones with a good deal of marrow. Knuckles, buy the way, typically need to be cracked, whereas the shanks are often sold in 2"to 3" pieces, so the marrow is already exposed.
A knuckle is a joint, like a knee or a hip. I think the neck with a front knee bone works the best. After you roast it for color, deglase the pan and boil with mirepoix, and your sachet. Start with cold water and simmer away. You could use any veal bone for flavor but the collegen comes the joints. I have used beef bones for stock but you cannot get demi from the older bones.
I hope this helps, this comes right from my "On Cooking book".
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.