Before reading this thread, I did not know that some WDW resort shops had distilled water for CPAPs "in the back" for guests who ask.
When distilled water is not available, I use bottled drinking water. Bottled water has minerals, but should others be just as "clean" as distilled water. My CPAP uses heat to evaporate water, so the minerals in drinking water stay in the reservoir.
The important thing is not to let the water evaporate completely. I fill the reservoir with more water than I need each evening and empty it completely each morning.
I'm not giving medical advice here. Check with your provider.