System Monitor is not part of ArcGIS, per se. As Xander points out, you get access to it through Esri Professional Services since they are the ones that develop it. For System Monitor, you pay a consulting fee to have them give you the software and help get it setup and working. If you are willing to pay for a tool, I suggest you do an internet search on tools for monitoring ArcGIS Server and GIS services. Although there are not a lot of tools out there, there are at least a couple nice packages I have seen.
If you search GeoNet, you will find multiple cases where users ask/suggest that Esri package System Monitor with ArcGIS Server. Given the cost of ArcGIS Server products, I think providing a robust monitoring tool with it isn't too much to ask, but Esri obviously thinks differently.