For those interested in 20er MIDI concertinas and 40er accordions,
both instruments are available online directly from the manufacturer,
Firma Watterott,  http://www.ziehorgel.de/produkte?lang=en 

I recommend you communicate with and order from the manufacturer;
contact them directly via email and pay through PayPal.  The
approximate delivered price for the 20-note concertina is 2,000 Euros.
The 40-note accordions are approximately 3,800-5,200 Euros delivered,
depending on the model.

As of this writing, I know of (10) 20-note concertina owners in the
USA (much growth in the last two years).  Again, communicate directly
with the Watterott Firm.  I purchased the first two concertinas in
the USA in 2009; please write me if you have any questions on these
instruments, thanks.

YouTube videos of interest:

  https://www.youtube.com/watch?v=cgoAE3MgDBE
 
  https://www.youtube.com/watch?v=W6s1VSZbfE4
 
  https://www.youtube.com/watch?v=a76_ng1SXVY
 

Gordie Davidson