Hi, I see what you mean, but I am afraid there isn’t one. Alternatively you can use e.g. a britta filter and not use expensive bottled water. Hope this helps.

A limescale filter on a humidifier is kind of a fool's errand. There will always be a massive amount of limescale produced, however, once the unit is dry it can be easily dusted off and removed.
The ultrasonic transducer itself doesn't get affected as its own frequency works a bit like a dental assistant's probe to ensure limescale doesn't accumulate.
The problem with this product is that its tank is too small for full power use while being too ineffectual at lower settings.