What is a good site to use for finding physical constants at times when NIST web resources are affected by government shutdowns?

There are multiple resources for finding physical constants (say, particle data, atomic spectra, or even special-function identities) which are generally hosted by NIST, and which form essential parts of the scientific infrastructure of multiple fields.

However, on occasion, these essential NIST-hosted websites can become unavailable for extended periods of time, particularly when NIST's funding is affected by government shutdowns in the US.

What are suitable alternatives which can be used in this situation?

Answer

The Particle Data Group is the only way to go.