Try and install Plasma from a live medium provided by several Linux distributions.

Shipping the latest code from Git

KDE neon

KDE neon builds Frameworks, Plasma and Applications on a continuous integration system.

You can use the Developer-stable version for builds from Plasma/5.x branches or Developer-unstable version for builds from master branches.

https://neon.kde.org/download

openSUSE

Argon and Krypton are openSUSE live images shipping the latest KDE software from Git master (unreleased) for Leap and Tumbleweed, respectively.

Ships Plasma 5.9

openSUSE

The latest stable Plasma version is available as a live image as part of the Krypton stable image:

32 bit version and 64 bit version

KDE neon

KDE neon builds Frameworks, Plasma and Applications on a continuous integration system.

The user editions build from released software.

https://neon.kde.org/download

Slackware

Alien BOB (Eric Hameleers) provides an installable Live image for Plasma 5 based on the development tree "slackware64-current". Login account/password are both "live". Use su or sudo to become root (using the "live" password).

Chakra

Chakra is a community developed GNU/Linux distribution with an emphasis on KDE and Qt technologies, utilizing a unique half-rolling repository model. The downloadable ISO features a bootable environment that allows you to test the system before installing it.

Ships Older Releases

Fedora

The Fedora KDE Spin comes with Plasma. New minor Plasma versions are pushed as regular updates for Fedora installations but live media may contain a slightly older release (depending on Fedora’s release cycle).

KaOS

KaOS is a rolling, independent, built from scratch distribution, focused on Qt and KDE. It makes regular ISO releases available, so any (new) user is not confronted with a large or complicated first update.

Kubuntu

Kubuntu 16.04 LTS and 16.10 support Plasma 5.8.5 through Backports.

Mageia

Mageia ships Plasma in Live Images.

Stable release 5 ships latest Plasma 4 aka KDE ;-)
Beta of Mageia 6 ships latest Plasma LTS : 5.8.x

You can update to latest Plasma in live mode, even without installing to disk.

openSUSE

openSUSE Leap ships Plasma 5.8 but without live images.