I believe real tortoise shell is illegal in the United States and a good deal of the world. I have seen it for sale in China and some other Asian countries. Several varieties of the sea turtle are endangered. Horn (Stag) is more readily available and not endangered.
Here is a pair of mine. Â They were made in Germany by, it looks like, "Lunor." Â All horn frames I have seen are from Germany, including those in the Ben Silver catalogue. Â Friedrich's of Palm Beach, Florida, has a good selection in horn and I was told there that tortoise is obtainable, although not stocked in United States.
Contact Tom Davies at email@example.com, he has a bespoke line available in numerous opticians in the U.S and Europe. I believe he does Horn but he told me he won't do tortoise shell due to the fragiilty of it.