What way can I find the beta / alpha repos for software?
Beta / alpha page from main site?
You need to add flathub-beta remote.
flatpak remote-add flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
(Or open that link in the browser and GNOME Software/KDE Discover should do it for you.)
Flathub only provides a beta branch.
GNOME and KDE host their own nightly builds: