1.- Newspapers (advertisements)
2.- Catalogues (musical instruments, toys or novelties)[see here, for instance]
3.- Shops (musical instruments, toys or novelties physical)
4.- 'Fakirs', or street agents, selling the nose flute among other novelties (see this post)
5.- Demonstrations (in concert halls, parties or world's fairs)
We already published some documents recruiting musicians for demos, and some other about the performances themselves. Here is an advertisement from a demonstrator looking for a job!
The Era, London (Mar. 21, 1928):
The demonstrations were held in different kind of places. Some were organized in music shops before Christmas, or even in concert halls.
Chillicothe Constitution (1912-11-27) and Santa Ana Register (1913-05-08):
The San Bernardino County Sun (1913-01-25):
The Republic - Columbus (1915-03-26):
Some demonstrations were performed during parties or musical shows as entertainement during other kinds of events. But privileged places for commercial demos were trade show and fairs, particularly the big World's fairs. In the streets of Chicago fair, Seattle or New York, there certainly were fakirs selling Humanatones. But the Humanatone Co. had also probably a booth here and there. We are sure that the Stivers had a stand at the Alaska Yukon Pacific Exposition (Seattle 1909)[check this post], but it's more than likely they also got one in other fairs.
I found an interesting picture of a stand for musical demonstration. It dates of 1939 San Francisco World Fair:
Unfortunately, it was not a Humanatone stage, nor even probably for a nose flute, but for the Hum-A-Tune. Yes, there was a nose flute called by this name (see this post), but it is impossible that this 1970s nose flute was demonstrated in 1939. It is not possible to see the instrument played by the musician, but it was certainly the Hum-A-Tune kazoo.
This instrument had many imitators (Hum-Al-Band, etc.) including the FanFare by Paul Brunner. There were also 'special editions' for the Hum-A-Tune, we know one made for the 1939 New York World's Fair.
Now, how did the demonstrators proceed? Well, it was a fair, so it had to be a real show! I found an extraordinary video document in which 15 seconds show a Hum-A-Tune demonstrator on stage, at 1933 Chicago World's Fair. The musician first plays the Hum-A-Tune, pretending to play a mini and damaged trombone… Then we can see him playing the Hum-A-Tune while bowing a totally wrecked violin (The whole document here). There is also a ukulele hanging on the side: