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:
sh latest-pepper-flash.sh
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.
Didn't notice that in the script we don't use any security by downloading updates from http instead of https exposing us to MitM attacks until today when I saw a report from ESET titled: Diplomats in Eastern Europe bitten by a Turla mosquito ( https://www.welivesecurity.com/wp-content/uploads/2018/01/ESET_Turla_Mosquito.pdf ) where Turla APT apparently injects malware on the fly in adobe flash player's update.
So I suggest to apply a little patch to the script by simply changing all http to https as soon as possible.
EDIT:
I verified analyzing the traffic generated by the script, even with the use of https the check of the version is in plain text because of a redirect in this page: https://www.adobe.com/software/flash/about/ -> http://get.adobe.com/flashplayer/about/ :D
The better solution is to use the suggest of @ohaessler ( is more faster and stable as he said, Thanks! ) and to use a string for verifying the version like this :
VERSION=$($SILENT_DL https://fpdownload.macromedia.com/pub/flashplayer/masterversion/masterversion.xml | grep -m1 "PPAPI_linux version" | cut -d \" -f 2 | tr , .)
Note the use of https and this time works and is full encrypted.