The 500 does a lot of different stitches without the cams - and most of the cams duplicate stitches that you can make with the built-in cams. I used to have a list of the cams that the machine couldn't duplicate, but I can't find it at the moment.
Singer had two basic styles of cam.
If you look at auctions on eBay, you're looking for the black "top hat" style cams - if you stretch your imagination, they could look like little hats. ;) The white ones will not work in the 500, nor will the black #23 basting cam made for only two or three machines in the 600 series of Touch & Sews.
You also don't want the other style made by Singer which are flat disks.
The only disks you're looking for are the black "hat" looking cams.
You can find them really cheap if you watch for a while - don't look for parts specifically for the 500 or 401 - look for parts for the Touch & Sews - they will usually sell for far less than the ones sold for Slant-O-Matics. :)