The only place I've seen them was from Chris Altland-York County Firelocks. I haven't heard if anyone has taken over since his passing.
I got some from https://www.peterdyson.co.uk/ I do not know what kind of import problems you might face
If I was needing mounts for a pistol with cast art, I would purchase wax's from MLBS and ship them to the foundry.
looking for something like this?
I used a firm to cast sterling mounts for a Jaeger Rifle last year. If I was needing mounts for a pistol with cast art, I would purchase wax's from MLBS and ship them to the foundry. Just a note, you would be advised to get two or three of each wax needed in case there are flaws. PM me if you want the contact for the foundry.
I'd be interested in a set if that helps the minimum required number or the price. I saw the castings Ron referred to and they were very nice.
Justin, the ones I got about 18 years ago, were actually Hallmarked sterling silver, did not actually peruse the site when I posted the link here, but will call his son in a day or two and let you know if he still does them