Welcome to the Vintage Sewing Machine thread. All of the Featherweight model sewing machines take the same size bobbins. The bobbins are available from many sources. Below are two of the sources for 221 bobbins.
http://www.sew-classic.com/
http://www.april1930s.com/index.html
According to the ISMACS website your Featherweight was allotted 13 July 1936, one of 15,000.
http://www.ismacs.net/singer_sewing_...l-numbers.html
If you want to find out all there is to know about Featherweights I suggest purchasing the book, Featherweight 221, The Perfect Portable by Nancy Johnson-Srebro. The book also has the 221 manual at the end of the book.