Hello Jan,  I think you'll find the actual diameter of your disks are
14.75 inches (or 37.5 cm).  Here in the U.K. we have Renaissance Disks
who are able to supply discs for just about all sizes of disc music
box.  They also have the facilities to copy old discs.  I have just got
a list from them for the 14.75-inch Symphonion and it lists 41 titles
available at a cost of around GBP 27.00 each plus carriage.

Their contact details are: -

  Renaissance Disks
  16 Penny Lane,
  Emsworth,
  Hampshire,
  PO10 8HE.
  United Kingdom
  Telephone: 0044 1243 372646
  Fax:       0044 1243 370299

Hope this helps!

Jeremy Brice,
Aylesbury, Buckinghamshire, U.K.