The new chipset is called Z390. It's an incremental update to Z370 rather than a complete overhaul, though it does offer a few new features. Specifically, Z390 introduces native support for USB 3.1 Gen 2 connectivity (rather than having to offload USB 3.1 chores to a third-party chip), integrated 802.11ac (Wireless-5, under the new naming scheme) support, integrated SDXC (SDA 3.0) support, and a newer Intel Management Engine (IME) firmware revision.
Most everything else remains the same. To that end, many Z370 motherboards have updated BIOSes available that add support of Intel's new Core i9-9900K, Core i7-9700K, and Core i5-9600K processors, and presumably additional 9th gen CPUs that have not yet been announced. For anyone building a system from the ground up, however, there are plenty of Z390 motherboards to choose from.
The cheapest one we found is Gigabyte's Z390 UD (Ultra Durable) for $129.99. It's one of nearly a dozen models by Gigabyte, including seven models under its Aorus brand for gamers. Or if factoring in mail-in-rebates, ASRock's Z390 Pro4 is a little cheaper, checking in at $124.99 after a $10 MIR. Like Gigabyte, ASRock has added 11 Z390 models to its lineup.
There are way too many models to list details specs and features for each one. In lieu of that, you can hit up the Z390 landing pages for each motherboard maker. Here are links to get you there:
As for the processors, the new 9th gen parts are all available to preorder now, and will release on October 19.