Mount a remote file system via SMB or NFS and it’ll work like any folder on your PC - but you need a network connection to access it. If you need offline mode, maybe create a folder /offline and replicate it to your hard drive with a bidirectional synchronization tool like unison or syncthing. As for providers, personally I use Hetzner Sotrage Box because the price and service looks good enough to me that I didn’t bother to look for alternatives.
KDE Discover is a GUI application for adding/removing packages from different sources