Adam Engst, writes for TidBITS:
Although the 27-inch iMac with 5K Retina display and iMac Pro both have 5K monitors, Apple is not currently creating a standalone 5K display, or actually an external display of any kind. When Apple dropped its 27-inch Thunderbolt screen (see "Apple Discontinued Thunderbolt Screen with No Change in Vision" June 27, 2016), the company worked with LG for a replacement: The LG UltraFine 5K Screen $ 1299.
So You could buy a LG UltraFine 5K screen, but you may not. This is because the availability of this screen appears to fall, with AppleInsider reporting that Apple Stores says it has not been uploaded for a while and is not available for storage at the store when ordered online. Fortunately, availability is dropping because LG replaces it with a new model, but LG can only run down again before it ends.
The 5K resolution Wikipedia page shows a small number of other 5K screens, including Dell, Philips and HP screenshots, but as far as I can tell, no one is currently for sale, apart from a handful of ultra wide screens with unusual aspect ratio as 64:27 and 32: 9. Also on this list is Iiyama ProLite XB2779QQS, but the page on Amazon says it is sent directly from Japan and has absolutely no ratings or assessments that are suspicious.
Good overview of the current state of 5K monitors for Macs, which apparently have not taken off because the industry has settled on 4K for now and 8K for the next generation.
By the end of the post, Engst assumes that the Apple brand display coming next year with Mac Pro will probably rely on Thunderbolt 3. However, I hope Apple can find a way to offer a 4K or 5K screen that works via Thunderbolt 3 and USB-C with support for ProMotion update rates of 1