You are on your Ubuntu system. Then you can try to add the Raspbian repository to
/etc/apt/sources.list. Add this line:
deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
You can only add it, but I recommend to comment all other lines in sources.list. They belong to Ubuntu and may confuse selection of the right package. Then do:
rpi ~$ sudo apt update
Never do an
sudo apt upgrade! It will break the Ubuntu system. Now you should be able to select and download the packages for Raspbian. Of course you can't install them if you do not run an arm processor on the Ubuntu system and you should never do it to not break your Ubuntu installation. When you are finished just revert settings in sources.list and do
sudo apt update again.