Adam Engst, writing for TidBITS:
Although the 27-inch iMac with 5K Retina display and iMac Pro both have 5K displays built in, Apple doesn’t currently make a standalone 5K display, or, in fact, an external display of any sort. When Apple dropped its 27-inch Thunderbolt Display (see “Apple Discontinues Thunderbolt Display with No Replacement in Sight,” 27 June 2016), the company worked with LG on a replacement: the $1299 LG UltraFine 5K Display.
So you could buy an LG UltraFine 5K Display, but you might not want to. That’s because the availability of that monitor seems to be in decline, with AppleInsider reporting that Apple Stores say it hasn’t been restocked in a while and that it’s not available for in-store pickup when ordered online. With luck, its availability is dropping because LG is replacing it with a new model, but LG could just be running down stock before discontinuing it.
The Wikipedia page for 5K resolution lists a small number of other 5K displays, including screens from Dell, Philips, and HP, but as far as I can tell, none are currently for sale, apart from a handful of ultra-wide monitors with unusual aspect ratios like 64:27 and 32:9. Also on that list is the Iiyama ProLite XB2779QQS, but its page on Amazon says it ships directly from Japan and has absolutely no ratings or reviews, which is suspicious.
Good overview of the current state of 5K displays for Macs, which seemingly haven't taken off because the industry has settled on 4K for now and 8K for the next generation.
Toward the end of his post, Engst assumes that the Apple-branded display coming next year with the Mac Pro will likely rely on Thunderbolt 3. My hope, however, is that Apple can figure out a way to offer a 4K or 5K display that works via Thunderbolt 3 and USB-C with support for ProMotion refresh rates at 120Hz. The 2018 iPad Pros can only connect to USB-C displays (not Thunderbolt 3), which is why I ended up buying a 4K UltraFine display that works with both macOS and iOS via a single USB-C cable. I want to believe that Apple's external display comeback will support both pro Macs and pro iPads; as the owner of a new Mac mini and iPad Pro used with the same LG display, an integrated Apple solution would be the dream setup.