In your area, go into the public library and look up the current copy of the "Business to Business" Phone book. Look under Machine Tool/Machinst Tool Supply and try contact them locally to see if they stock sets of lowercase stamps. Most tool places have the upper case sets and they tend to be relatively inexpensive because that is what most folks will use if you are doing single letter stamps.
Alternately you may consider a panagraph or engraving machine, but those too are generally not cheap unless you manage to get one used. You could use a little grinding hand engraver and do the lettering by hand too. Harbor Freight has those little electric ones for under $20 I think.
There is also the possibility to do chemical photo etching, but I am not sure how silver reacts to that and I have only used it on tool/stainless steels (primarily marking knives).
That probably doesn't help too much but maybe it will help,
Robert