AFAIK, you’re not going to find them either. Signet rings need to be
forged. Engraving cast rings often will often reveal hidden
porosity. Forged rings don’t have that problem, and the worked metal
takes the tool better as well. Quality signet blanks are drop forged.
Unless you’re doing multiples forge the shank. If you are doing
multiples, cast them. Either way, you won’t have to modify your
design to fit someone else’s pre-made piece.