I have a vintage Singer 15-91. It uses a different sized bobbin than a Featherweight 221, but the class 15 bobbins are still made just like the Featherweight bobbins. Most of the newly manufactured 15 bobbins are made from a lighter weight metal, which I finally figured out my machine doesn't like. Even though the new ones fit in my machine, they rattle around in the case more than the original. This slightly affects the stitch quality. I just use my one original bobbin. Other people are able to use the new size 15 bobbins with no problem.
There are some e-bay sellers that sell vintage parts for Singer SM. I really need to buy some old bobbins and throw out the collection I have bought trying to find a new brand that works. If you find that your Featherweight doesn't like the new ones, you might search on E-bay for a supplier of vintage parts. I think there is a Yahoo group devoted to Singer Featherweights, you might join and find a source there.