You could probably pick up a reverse osmosis unit from a good pet store for about €150 which will give you about 15 gallons per day.
Might be cheaper in the long run.
What do you use the deionised water for (I’m new to photography). The reason why I ask is I keep a marine tank and have a RO/DI unit and have this stuff literally on tap!
My guess is as there’s zero TDS you can use it to clean lenses?
De-ionised water is used for processing film and darkroom printing.
The only issue with RO water is that it’s pH can swing wildly has there is no electrolytes to buffer it so not sure how that will affect the pH balance of the developers etc
RO is nearly as pure as distilled water