In my office environment we have several systems that are not connected to the internet. Using apt-mirror I am able to make Ubuntu repositories and PPAs available to all the machines on this "offline" network.
Is there something similar for snap?
There is a proxy store available. This runs on your own infrastructure and does the caching you require, although 100% offline is, according to the docs, currently in closed beta. Note that this thing is a commercial offering, i.e. it costs money, and last I checked Canonical doesn't publicize exactly how much.