Option 1:
You can set up your own repository and use a machine with access to populate that repository from the online one. This can be as simple as a UNC share set up properly, then you can register that repository on the client machines e.g.
Register-PSRepository -Name MyRepo -SourceLocation "\\fserver\share" -InstallationPolicy Trusted
All your isolated machines can use this repository by requesting the module with the repository name:
Install-Module -Name MyModule -Repository MyRepository
There's plenty of documentation on how to set up and publish to your own repository.
Option 2:
Powershell modules are contained in folders under one of the module paths you can get from $env:PSModulePath variable.
You can download them onto a machine with access and copy them to the target machines by other means.