I have found this interesting site that has the history of them, including a few pics. About 5 yrs ago, I was gifted a pair of earrings and a ring of this filigrana charra, but didn't know the history until now, so thanks for asking your question here
And I couldn't find where to buy them yet. Maybe you can email this site to learn where one can buy them.
buena suerte http://www.artesaniamendezlaalberca.com/