My plan is to etch some brass and use that to stamp silver sheet.
Do you mean emboss?
Buy Susan Kingley's Hydraulic Die Forming for Metalsmiths. I think it
is not available on amazon, but will be available from Rio, etc.
And hop on over to http://www.bonnydoonengineering.com and join their
forums and read up on their site.
If you mean embossing you will need some urethane, preferably in a
range of hardnesses (durameters) but one will do in a pinch.
Hard to Find Tools for Metal Clay