Donna,
There is also a Singer 503 that looks 99% exactly like the 500A, but it doesn't have the internal camstack. Just FYI.
Here is a GW auction of a box of top hat Singer cams that will fit your machine:
{
http://www.shopgoodwill.com/auctions...-21152283.html }
I've picked up quite a few of these accessories boxes for the components and cams. I have quite a few slant shank machines.
Joe