These are what they usually look like these days

https://www.amazon.com/dp/B000VPLIDC

Though you can pay extra to get an unpainted version (what a racket! pay more for less work at the factory lol)

https://www.amazon.com/dp/B01H0G63YS

They're not brass, because they're still made the old way by rolling a plate of metal around a mandrel. But, I'm not sure if they're exactly tinplate or not. Might be. With a bit of effort you can fold one in half with your bare hands (and destroy the whistle in the process).