Joanne,
Sew-Classic {
http://www.shop.sew-classic.com/ }, April1930 {
http://www.april1930s.com/ }, and others have Walking Foot attachments. I have four right now, one generic ZZ that my wife got from eBay many years ago for her Bernina, it came with an adaptor, two marked SINGER, a slant shank and a vertical shank ZZ and the most recent one from Sew-Classic that doesn't work near as well as the others.
Several previous posters have the same WF as I got from Sew-Classic and they work good, so I'm going to try mine in a couple other machines then if still doesn't work, I'll return it.
But I'm going to be looking for a SINGER straight stitch WF in the future.
These old machines are heavy, but I prefer them to the lighter ones. They don't move around when I'm using them.
Joe