You need to get on a Snow Leopard system, log into the App Store with your Apple ID, then download the Lion installer. The re-download is free. After the installer is downloaded it will start automatically. Now, quit the installer. You can now make a copy of the installer that you will find in the Applications folder.
Now, you also may be able to get this to work: Open the App Store application. Hold down the OPTION key while clicking on the Purchases icon in the toolbar. This is supposed to activate the Install button to the right of your Lion purchase entry. Clicking the Install button will then re-download the Lion installer.