My Singer 328 took the flat, black cams. My Singer 503 (Rocketeer) takes the black, "top hat" cams. These cams seem to be the easiest to locate, especially on eBay. The 503 won't, however, use the white "top hat" cams. I believe those are for the 600-700 series, might be mistaken on that.
i love my 503 - the stitches are beautiful! I haven't tried sewing multiple layers on it, but it seems powerful enough to do it. Good luck in your search!