Where are apt-get files stored?

The .deb-files you have allready downloaded are stored in /var/cache/apt/archives/.

Not an explicit answer to your question, but have you considered setting up an apt proxy? I use apt-cacher-ng (apt-get install apt-cacher-ng :D). Prehaps set this up on the VM host, and tell the VMs (and the host) to use this as their proxy (as simple as adding a file (eg 02proxy) to /etc/apt/apt.conf.d/ containing

Acquire::http { Proxy "http://vm host ip:3142"; };

That way you can just do apt-get upgrade without having to manually copy file around. When one computer downloads the debs they will be stored on the proxy for the next computer that requests it. Can handle multiple releases and different architectures, etc

Just a though :D

/var/cache/apt/archives is where they should be stored.