Welcome Gerry.
To locate the Sat Nav disk, go to the boot, locate the jack and just to the right you will see a black removable panel. Behind this is the eject button for the disk.
As far as I am aware, the cheapest update disks can be found at the link below. They are genuine Lexus disks and probably much cheaper than main dealer supplied.
You will need the various serial numbers from the disk when you check if an updated version is available for you.
I have the 2006 version which has plenty of updated info but the graphics etc are no different if I remember correctly.
http://www.lexus-mapupdates.eu/