A flatpak package is available in Pisi 2.1 and newer. To install it, run the following as root:
sudo pisi it flatpak
Flathub is the best place to get Flatpak apps. To enable it, run:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To complete setup, restart your system. Now all you have to do is install some apps!