Any musician guitar, bass, keyboard 'volume pedal' that has a standard pot in it can be modded with a pot in the value and taper required.
Tip: try the pot you think you need first before fitting,
and buy some exact value spares once you've got a winner
If you can't find a volume pedal, gutting out a stuffed wah or swell pedal will do the same job,
as long as it's mechanically ok
i.e. the pedal and gear parts work
Some pedals have opto-couplers? instead of pots, watch out for that
