Hi Jon,  I know you can order strips for the 30-note Sankyo
hand-cranked movement from:

  http://www.musichouseshop.com/cgi-musichouseshop/sb/productsearch.cgi?storeid=*1875409baa5ed4ecde68aee24120 

I'm not sure if the strips for the 20 note are the same or a different
width.  However, if the 30-note strips are a bit wider, you can probably
carefully cut them down to the size you need.

Alternately,  please contact Ron or Kandi Wasser at
info@themusichouse.com.geentroep [delete ".geentroep" to reply]
and ask if they can get the 20-note strips for you from Sankyo in Japan.

Please let me know if I can ever be of further service to you for music
box parts or repair/restoration services.  Best of luck in your quest.

Most sincerely,
Don Caine - Proprietor, The Music Box Repair Center Unlimited
Lomita, California
http://www.musicboxrepaircenter.com/ 

 [ These links may be useful:
 [   http://www.musichouseshop.com/store/index.html 
 [   http://www.musichouseshop.com/store/Musicboxes4U.html 
 [   http://www.musichouseshop.com/store/njdMBM30hp.html 
 [ -- Robbie