Ev-entz has a great selection of the sshhmute practice mutes.

This tenor trombone mute is made from lightweight, yet hard-wearing, ABS plastic with no parts to break or fall off. Perhaps one of the primary benefits of the sshhMute design is the very small amount of back-pressure it creates.

This allows for better air supply, while still opening and developing the throat muscles. The mute also remains in tune and faithfully accurate throughout the entire range.

Basically it's like practicing without a mute (but a lot quieter)

Price: £52.50

Go to: https://4barsrest.shop/product/sshhmute-tenor-trombone-practice-mute/