The remote control for that unit is model number RC780SR
You can get an original factory replacement remote but it will probably be expensive.
Here is a link to one distributor that sells them:
http://remotes.com/store/marantz/rc780sr.html
Just use your mouse to click on the link, which is the underlined and highlighted text just above. This will open a new web browser page automatically for you and allow you to view the information at the website indicated.
You are probably better off spending that kind of money on a nice Sony or Logitech Harmony universal remote that you can program to work with any of your home electronics.
Anyway, thank you very much and good luck.