What @browneyesvictim said .... St. Louis ribs are just highly trimmed spare ribs. With a little practice you can cut them down. Trim the upper chine/flap down to where the "knuckles", or joints of cartilage are. Trim the back, removing most fat, and remove the membrane. Now you have St. Louis ribs.
looks delicious! I haven't been able to find any St. Louis cuts around here in a long time. Only baby backs, and spare ribs. Baby backs are too lean, and spare ribs too fatty for my preference.