Special material needed

I know a guy who builds farm equipment professionally, he scours the hardware stores looking for bits he can use, such as washers, shims, lock washers for such components as discs and choppers. Here are a couple:

You can google search images of "lock washers" to get a good idea of whats out there. 

First, to the forums!  Second, if you are looking to find or make domed or dished parts, consider using a dapping block and puch set.  They can be found from many tool suppliers, and come in wood, brass, steel, and perhaps some other materials.

Here is a photo of what they look like:

HTH!

As far as chain goes, there are lots of companies that make scale chain, just go to Greatmodels.com or Squadron.com or any other model store and search for chain.  The only problem is that you don't get very much (3 inches or so). Or you can go to Hobby Lobby or Michaels and look in the homemade jewelry section and find it in larger quantities, I had to look for awhile to find chain that looked "right" to me.

I assume by discs you mean discs that have a shallow concaved shape for a disc plow?  I'm not so sure about where to get those in that shape, you could cut plastic or thin metal discs with a circle cutter that are used to cut mat board and plastic, and then maybe use heat and a form to shape them.  I'm sure somebody will come up with a better solution.