When you download an app, the installation process unpacks and removes the original installation file, so after the installation it is no longer around for retrieval.
It may be possible to grab the file just after it has been downloaded, but before the installation removes it, but I'm not sure if on Symbian^3 it sits temporarily in a public (accessible) folder on the device, or if it is in some private (inaccessible without AllFiles capability/privileged app that can access it).