Get Plasma Bigscreen
Plasma Bigscreen is now released as a part of KDE Plasma, and is available as a package in Linux distributions.
Some distributions ship dedicated Plasma Bigscreen images, providing an out-of-the-box experience without any extra installation steps.
You can check the status of Plasma Bigscreen on the Matrix channel or the KDE Invent repository.
Fedora
Fedora provides the plasma-bigscreen package in their repositories. There is no dedicated edition just yet, so you will need to start from another (ex. Plasma Desktop).
postmarketOS
PostmarketOS (pmOS) is a distribution based on Alpine Linux for both embedded devices (phones, SBCs) and desktops. View the device list to see the progress for supporting your device.
Plasma Bigscreen is available as a UI when building an image (following the pmbootstrap image builder guide). It is not yet available as a ready-to-download ISO.
KDE Neon
KDE Neon has a development version of Plasma Bigscreen in its repositories. This image, based on KDE neon, can be tested on x86-64 based computers.
Building it yourself
Developers use the kde-builder utility to build and develop Plasma Bigscreen on most distros (with reasonably new dependencies, like Arch and Fedora). See the wiki page on how to set this up.