How do we get Required RVR or Actual RVR for an instrument approach?
Current RVR is obtained from the METAR, yes, which in turn is obtained from transmissometer(s) on the airport site.

Minimum allowable RVR is obtained from charts (chart legend from Jeppesen, PDF), more specifically the instrument approach plate for approaching an airport. And on the SID chart for departing.

Example:



R is RVR in hundreds of feet, V is Visibility in miles, in this example. Units can change and will be denoted, check full PDF for all examples.