I just came across this website:
http://carsalesbase.com/us-car-sales-da ... z-b-class/
Which shows car sales, in particular that URL shows US electric B-Class sales for the last few years.
I did a screen grab because there were too many numbers to copy & paste. So it looks like total US Sales averaged 50 cars per month from late 2015 through early 2018. The peak in 2014 was 278 cars in May with numbers around 200 or so otherwise earlier that year.
I knew there weren't many B's around, but it's kind of stark to see the real numbers. I'm amazed my local dealer services it at all, but perhaps it's because they also sold Smart Cars and there were some electric models of those too.
But don't expect any 3rd party products for this model, not with numbers this tiny! Sales of Bolts, Volts and even i3's are at least 10x greater.