I have a customer site with servers isolated from Internet access. Some are Server 8r2, some are Server 2012r2 and a few are Windows desktop 7. Has someone developed a methodology to side load most of the popular PowerShell modules onto these isolated systems?
If they have no internet access, then someone is going to have to put those modules somewhere on the network (or onto the images that are used to build those servers.) How you get the modules onto the machines themselves is entirely up to you; that’s just a file transfer. Many, many ways to skin that particular cat. 
It varies. There is no “most popular” list. You load according to need. For instance I don’t have SCCM in my environment, so I’m not looking to load its modules. But I want RSAT to administer Active Directory by my IT Support Team, not company-wide.
And how “isolated” are these systems?