Gemesis, aka Brilliant Earth, is the biggest name and they have lots. If you go with them I would recommend shopping around for a good local dealer (or even an appraiser) who can help you evaluate them. Diamond Foundry is much smaller but also has them.
If you want the absolute best of the best, Good Old Gold has a few that they have vetted, and will even work with you to do custom cutting, which would be better than, I dunno, 99.9999% of the diamonds out there.