for application icons right click on an app in the Applications folder and choose "Show Package Contents", open the Contents and Resources folders and the .icns will be in there. for Mail its "app.icns".
im not sure where the internal hard drive icon is but you could bring up the Get Info window on your hard drive (select and press command+I), click its icon in the top left (will highlight blue) copy it and then open Preview and press command+N.