Package server - failover if package not on package server

How do I set up a package server so that if a package is not available on the package server it will download automatically from the NS.

The issue we have is that some packages were set up not to go to a package server or the package itself has not been copied to the package server (awaiting the package server distribtution task) this result in the client continually retrying download.

Is there a way to force clients to get a package from the NS if it first fails from the package server?

Another reason for this is our Sys admins team have advised us that the package server hardware may be subject to outages!!!

Catherine