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