I have just bought a Transcend 365 machine for travelling however it says distilled water MUST be used in the humidifier or it could cause damage .
I have had a Cpap machine at home for 13 years and never used distilled water, just cleaned the tank out with diluted white vinegar when needed to remove scale.
Where do get your distilled water from as I can’t find any supermarket or chemist that stock it just on line stores.
or is it worth buying a distiller. Which is about £45.
Sorry David as missed your question. To be fair all the manufacturers say to use distilled water in the manual, but as you've discovered it's hard to get it here. I live in a very hard water area so I do purchase it but it's not cheap. I get 10 litres at a time https://smile.amazon.co.uk/gp/product/B06WCZM6Q2/
£45 sounds cheap for a water distiller and it's better to get one from the UK so you can easily buy the filters and other accessories.