The GDI and Nod discs from this do work, but you can search for more discs on archive.org and other places to get other disc images for The First Decade, Covert Operations, etc.:

https://archive.org/details/command-and-conquer-tiberian-dawn

You may also find discs here:

https://cncnz.com/features/freeware-classic-command-conquer-games/

OpenRA's wiki mentions these locations:

https://github.com/OpenRA/OpenRA/wiki/Game-Content#downloading-original-game-cds

To install the discs when prompted, mount the isos to one of these locations on Linux:

/media/openra
/media/{user}/openra
/mnt/openra

https://github.com/OpenRA/OpenRA/wiki/Game-Content#linux

You can use this tool to mount the disc: https://wiki.archlinux.org/title/FuseISO

Make sure the directory has appropriate permissions to mount like so:

sudo chmod 777 /mnt/openra

Then mount with FuseISO:

fuseiso /path/to/iso /mnt/openra

In OpenRA, click Detect Disc or Installation (or Retry if you are already on that page) and it should be able to detect the disc and extract the files.

You can unmount and delete the directory (or mount and install other discs first before deleting the directory, of course) when OpenRA finishes extracting the files:

fusermount -u /mnt/openra
sudo rmdir /mnt/openra

I am assuming for the non-disc archives, you would be able to either copy or symlink the extracted files to one of the supported mount points, and the game should be able to detect the content. Downloads are taking a while so I haven't been able to test yet to confirm.

OpenRA does let you download the base game files plus some other content and patches from the internet.