Source for calves feet in Brooklyn
For stock purposes I doubt if it matters whether you use calves feet or cows feet. For cows feet I'd check markets that serve Hispanic or Asian customers. Cows foot is a highly desirable addition to Mexican tripe soup (menudo). Even pigs foot would work as a concentrated source of gelatin (or other parts with lots of skin). Asian markets also sell tendon, which you'll see listed as meat option for Vietnamese Pho.