Rodney is correct about the 500 being a slant shank machine.
I have found that generic or aftermarket walking feet do not always work properly on Singer machines. Try to find a Singer Even Feed walking foot for the slant shank and it will work correctly.
Also the Singer presser bar has slightly different dimensions than other brands, so sometimes you'll find aftermarket feet and accessories that won't line up properly with the needle. You've got to be careful with what you buy.
GoodWill.com has a ton of accessories most of the time. So there are more than just eBay or Sew-Classic.
If you stick with Touch & Sew accessories for the ZZ machines you'll find 99.7% of what you'll want for your 500.
Joe