As the question say, i want to know if there is a way(s) to have an alert when a standalone splunk environment get down
partly it depends on what kind of "go down" we are talking about. Obviously, if we are talking about a major crash that "bricks" the environment, if there is only a single standalone splunk environment, then that environment cannot send you an alert if it goes down. Because it's down.
You would have to look to the operating system, or an external entity that can poll splunk periodically and notice a missing response.