Right now you can buy the Talos II motherboard (or a complete prebuilt system) from Raptor Engineering, and along with it you can have a single or dual processor setup of up to 22 cores each with 4 way SMT (essentially hyperthreading). It's still early days but I hear it's competitive with x86_64 at certain things and runs Gentoo well.
As a bonus most of the firmware of the components on the board are open source as well, and just a git clone away.
(Not sponsored by them, but I've considered such a system for myself)
From the article: “The Talos II server with dual IBM POWER9 22-core processors ended up delivering performance around that of the EPYC 7551 previous-generation Naples processor. But overall the Talos II POWER9 had quite a respectable showing compared to the x86_64 CPUs.”
I got to use POWER8 and POWER9 when the company I was working for partnered with IBM. My main reason I liked them was: they’re very cool, heh. The other reason was I quite enjoyed the AltiVec instruction set was a great fit for the work I was doing, the integer vector instructions were awesome. Nothing you can’t do on X64, but was nicer in a few ways.
As a bonus most of the firmware of the components on the board are open source as well, and just a git clone away.
(Not sponsored by them, but I've considered such a system for myself)