You can store about a day's worth of diesel on-site and have agreements in place to have daily refills in cases of emergencies.
If you can't get gasoline within a day's drive then there's bigger problems in the world.
In many places going over what you can store might put you over emission quotas and you would have to shut down anyway. I'm familiar with one incident at large DC which had fuel left and could easily get more, but only had few hours before they were required to shut down by EPA.
Natural gas generators.
Looking at multi fuel generators is worth it too depending on your setup.
For example some can run gas/propane/natural gas
But if you need enough backup capacity to survive something a multi-state, multi-day blackout [1] that probably gets expensive.
You wouldn't need that for a premium erotic call processor, but a 911 call exchange might, for the portion of their workload they can't pass off to another exchange.
[1] https://en.wikipedia.org/wiki/Northeast_blackout_of_2003