If you use Ubuntu or a derivative distro, issue the following to install an appropriate version of Flash:
sudo add-apt-repository "deb http://archive.canonical.com/ubuntu `lsb_release -cs` partner"
sudo apt update
sudo apt install adobe-flashplugin
If your distro does not provide a copy of Pepper Flash that works with Vivaldi, this script will download and install it for you. To use, click on the "Download ZIP" button listed on the GitHub Gist page and then unpack the .zip archive locally. You should now have a directory containing the file "latest-pepper-flash.sh".
Open a terminal within that directory and run the script like so:
The script will fetch and install the latest Pepper Flash. From time to time (perhaps once a month) you should re-run this script and if there is a newer Flash it will upgrade your installed copy, otherwise it will just confirm that you have the latest version.