Marantz, and other most other manufacturers, have generally accepted the policy of not selling parts to individuals. They sell only to authorized service centers but the service centers also do not sell parts to individuals. Too many people asking for parts that lack a parts number.
Rotary selectors are somewhat generic, you might find one at https://www.parts-express.com/
Otherwise, with a fifteen year old receiver, your best bet would be to find a dead receiver and salvage the part.
If all else fails, you can probably adapt another selector switch for the OEM part; https://www.google.com/search?q=rotary+selector+switch&rlz=1CAACAY_enUS754US756& oq=rotary+selector+switch&aqs=chrome..69i57.5274j0j1&sourceid=chrome&ie=UTF-8