Man, I would love to find Baby Backs (or loin Backs) on sale for $3.00/lb! Around here Baby Backs/Loin Backs are usually $3.99/lb on sale; double what spares go for.
So, I just get the whole spares & trim them to a St. Louis cut myself.
I know a lot of people throw the trimmings away but we use them. The skirt flap & end trimmings get cut & frozen for stir fry, or ground for sausage. I'll either save the rib tips for later cooks or cook them alongside the ribs and they disappear just as quickly.