You won’t find Carre cuts at most popular diamond retail stores and you’ll find your options limited if you’re after this special cut.
Dock, some smaller retailers who specialize in bespoke services may be able to source you Carre diamonds if you contact them.
Another good option is to check sites like Etsy and Amazon, where there are a number of unique Carre diamond rings and loose stones. Etsy in particular has a good range of highly valuable antique Carre diamond rings.