If this link:
https://jmeter-plugins.org/get/ doesn't work for you the alternative way of obtaining the Plugins Manager .jar file is downloading it from the
Maven repository, choose the latest possible stable version, download the .jar file and drop it to "lib/ext" folder of your JMeter installation.
Just in case here is the link to JMeter Plugins Manager 1.6 (latest version as of now)
And last but not the least, you can always get
the source code from Github and compile your very own version yourself, if you have
Maven installed and configured running simple single
mvn package command will compile and create the .jar file
JMeter restart will be required to pick up the Plugins Manager.