I tried tap water last night, this time boiling it first, and it seemed to work fine. My understanding from reading is that the main risk of tap water is that mineral deposits will build up in the humidifier tank. But it didn’t seem to have any negative impact on my breathing.
EDIT: London Drugs just dropped my order for distilled water. So it seems even their warehouse is now out. So I might be stuck using tap water from here on out. I did find this on the FAQ page of my CPAP clinic:
If I run out of distilled water, is it OK to use tap water once in a while?
Yes. It is recommended that you boil the tap water before using it in your humidifier.
So it seems boiled tap was is okay at least temporarily. I fear these shortages we’re seeing might drag on for a while, though.