University of Michigan has a 2016 video describing the original taxi horn tones of the Gershwin score. [Updated 11/19 See also new Taxi Horns post.]
Reader Jan (see comment below) shares an insight into the notes used in the original recording. That lead to this article (lots of interesting details): 1929 Gershwin Taxi Horn Photo Clarifies Mystery. It would seem the mystery of the actual tones used in the original recording is solved!
A comment from Bill Schuetter today provided the missing clue to discovering a source for tuned squeeze horns. Bill provided a link to a YouTube video of the LA Philharmonic performing Gershwin’s ‘American In Paris’. There they are! and certainly orchestras around the world would need these to perform Gershwin. I found a few sources. See links below.
[Update 5/11/15] There you go! Owner Russ Knutson, of Chicago Percussion Rental comments below to point out that the horns in this shot came from his shop.
Certainly not cheap to purchase. That’s about $750. U.S. From Kolberg in Germany.
Most orchestras rent the set of horns. In the U.S. often from Steve Weiss Music outside of Philadelphia.
Also from LA Percussion Rentals. FSTJ Percussion in Quebec, Canada.
Chicago Percussion Rental provided the 4 taxi horn rental
which are in the picture on your website.
Our white Chicago Percussion Rental sticker
is clearly on the side of the black wood frame.
We rented the 4 taxi horns for the American In Paris performances pictured,
and you don’t even mention Chicago Percussion Rental on your website.
Please correct this error on your website
and state that Chicago Percussion Rental
rents to symphony orchestras
the 4 tuned taxi horns needed for American in Paris.
I thank you in advance for correcting your website.
Russ Knutson owner
Chicago Percussion Rental
847-729-4177
http://www.chicagopercussion.com