What piece of veal should I ask a butcher/meat shop in the US if I want to do stew dishes like "blanquette de veau" or "veau marengo"?
Veal and beef cuts are completely different, the terminology is not the same at all. With veal the tendron is part of the breast, which includes the foreleg and the front of what would be considered the flank on a full-sized animal, in other words the part that does the most work. The tendron cut includes the lower-front part of the ribs. As for what to ask a US butcher for I'm not sure, I'd ask for veal foreribs and hope that they get what I'm after. 

Here are some definitions that may help.