Try Empire silver online for sterling cups and flatware. Also, Nelson and Nelson has a vast array of anything in sterling you could want.
Replacements.com is a huge source for silver as well.
If Rio Grande has 1/4 hard 24k you can roll it into a coil, heat it, and quench in water to soften it up. You might also consider a draw plate so you have the capability to reduce the diameter to different sizes.
Most manufacturers are forthcoming about the types of stainless they use in a particular firearm and will generally give you a Rockwell hardness as well. A telephone call is a good way
to avoid a potential catastrophe.
The base receiver is a zinc based metal that in its unfinished state is white like steel. They are plated with several metals to handle the final gold plating. If you put a graver to the metal you pierce the plating and run a serious chance of the plating flaking off the base metal.
I have done some Kimber stainless autos without difficulty. It might be worth a call to Kimber
to speak with their technical people about the steel in their revolvers. They are usually forth coming about the steel types and Rockwell hardness.