Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Whistle (company)
The idea for Whistle grew out of current CEO Ben Jacobs' experiences on petcare accounts at Bain & Company, and was developed with COO Steve Eidelman while researching the petcare industry for venture capital firm DCM. They hired Kevin Lloyd as CTO and the three founded Whistle in 2012. The company started with 20 employees and raised $6 million in a venture funding round led by DCM. Other investors included the former CEO of Mars Petcare and the president of Humane Society Silicon Valley.