For the cams, I think your best bet is probably eBay. They probably don't come up very often. However, if you get an eBay sign-on, you can place a search on eBay so they email you any time something new is listed that meets your search criteria. You might want to put in something like Elgin sewing machine for your search criteria instead of just Elgin cams. Sometimes people don't know what cams are, and sometimes they might be included with a parts machine for cheap.
For the walking foot and SID foot, you probably just need to determine what kind of shank your machine has. Most older machines are short-shank and you would purchase generic short-shank feet. Just be careful on the walking foot, as some generics aren't made very well.
You can Google to find other sites, but here's one that explains how to determine the shank size of your machine.
http://www.scribd.com/doc/24616337/H...our-Shank-Size