The FAA publishes lists of airports that have ILS and VOR approaches designated as backups if the GPS system is disrupted. These MON (minimum operational network) airports are part of FAA’s plans to decommission about 30% of the existing network of VORs by 2025. The MON airports are not the only airports that offer ILS or VOR approaches. But the procedures at MON airports do not include PBN elements (i.e., GPS).
As the FAA’s MON program website notes:
The VOR MON is further designed to allow aircraft to proceed to a MON airport where an ILS or VOR approach procedure can be flown without the necessity of GPS, DME, ADF, or Surveillance. Of course, any airport with a suitable instrument approach may be used for landing, but the VOR MON assures that at least one airport will be within 100 NM.
The AIM adds:
Any suitable airport can be used to land in the event of a VOR [sic: GPS] outage. For example, an airport with a DME−required ILS approach may be available and could be used by aircraft that are equipped with DME. The intent of the MON airport is to provide an approach that can be used by aircraft without ADF or DME when radar may not be available.
AIM 1-1-3 (f)
Lists of MON airports are sorted by state, which can be misleading at first glance. The state boundaries aren’t especially important–the intent is to provide a MON airport within 100 nm. And even if a widespread GPS outage occurs, you could probably get ATC assistance to find a suitable airport with weather above LIFR minimums. MON airports are truly “last resort” airports.
MON airports are shown on IFR en route charts. See New Airport Info on FAA IFR Charts. More information about the VOR MON program and MON airports is in AIM 1-1-3 (f) VHF Omni-directional Range (VOR). See also this Charting Notice from May 28, 2019.
You can find lists of current MON airports in the Chart Supplement (formerly called the A/FD) books for various regions of the U.S. For example, here is the list of MON airports in the Northwest.
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