The mute you really need is a simple toilet plunger (remove the stick, or just hand it back to the store clerk saying "I don't need that"
![Wink ;)](./images/smilies/icon_e_wink.gif)
).
You hold the plunger with your left hand against the bell (rest the bell on your left palm) and manipulate it to cover the bell or not. Listen to what happens.
You can even try this with your hand but on a large bore trombone or a bass trombone this doesn't work that well.
And here's somebody with one:
![Image](https://i.ytimg.com/vi/mEhgXrtKljc/maxresdefault.jpg)