Beta / alpha page from main site?

What way can I find the beta / alpha repos for software?

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.)

Yes, now just the alphas…

Flathub only provides a beta branch.

GNOME and KDE host their own nightly builds:

https://wiki.gnome.org/Apps/Nightly

https://community.kde.org/Guidelines_and_HOWTOs/Flatpak