AFAIK Yast doesn't keep the rpm files after it installed the packages.
To be able to keep the rpm files, it is necessary to download them manually before installing them either with Yast or by the rpm command in a console.
You can also use the Smart package manager which allows to keep the rpm files, contrary to Yast.
If the packages are already installed, you can download the rpm files for example with wget or with the download function of a browser, and keep them somewhere.
I think your best choice is to use the Smart package manager.