Pmarklin,
Welcome to the QB.
What is wrong with original Singer feet and accessories?
Anecdotal experiences: I have 5 slant shank Singers. A 401a, two 500a's, a 4622a and a T&S 778(?). I have tried many aftermarket "modern" feet on them that just would not fit properly or did not work any better than the originals. The needle slot in the foot was off side to side or front to back. Or the shank where it screws to the pressure foot bar wasn't machined right so the foot twisted. Or the thing was made from cheap pot metal and wouldn't hold adjustments. Not all feet and attachments are made to the Singer standards of your machines vintage.
Singer made just about anything you'd ever need for the slant shank machines. The most prolific source for these feet is Shopgoodwill.com auctions. Lots and lots of slant shank feet and accessories to be had there.
Sew-Classic is another place and there are more. But look for originals first. Just remember the old saying: "Just because it's new doesn't mean it's good".
Joe