There isn't one in most modern airliners.  It is a tied and computerised function to the electrical busses.  On battery power, avionics are on a needs basis (only really necessary avionics receive power), once a more reliable source is provided (APU, for example), all avionics should be on and ready, save those that are dependant on the IRS/FMC.

In short.  It's not a switch you operate as a pilot, so you don't need to look for it.

Happy flying,

Kyle

if to key over the blank area, just left of the autopilot controls, and left click the radio obs, and frequency box will come alive, hopefully with your version also.