SewClassic have
this adaptor. I don't know exactly how it works. I guess there are Singer sets for it. I was never that happy with the snap on feet for my Bernina, the original feet are better and they are easy to change, so I never searched much further in that area. However, there are various new slant shank feet available if you aim at a specific purpose. There's quilting feet made, like the
Westalee ruler feet.
Here's a metal snap on adaptor, on ebay there's at least
two different plastic versions.
Here's the second type. Most accessories are available for the most common vintage models and brands. If you bother to search a bit, you will find lots of stuff. For some models you need to search a bit more to find alternatives, but even for the early back clamp 66 I have seen an adaptor to general low shank feet.