|Syntactic Confectionery Delight
Some excellent advice above.
From your description, though, I gather that you do have internet access but through a proxy (an "air gap" as you call it). If that's so, you can still use ppm, but you have to tell it where the proxy is. The bottom of this page has the details. Basically, you need to set the environment variable HTTP_proxy to the address and port number of your firewall, and you should be good to go.
I work behind very restrictive firewalls at some of my clients, and I've used this technique successfully everywhere.