Check with the supplier or manufacturer of the CPAP machine.
Some people have posted in the past that they just used the machine without water for their WDW trip (like several posters on this thread have posted that they personally did not need the additional humidity in Florida).
Other people have posted in the past that the reason for using distilled water was to prevent mineral build up. When they checked with the manufacture/supplier, they were told that using tap water all the time was not recommended, but for a limited time (like during vacation), it would not be a problem.
Another thing people have posted was that they ordered water to be delivered to the resort. I don't remember where they got it, but here are a number of grocery services that take orders and then deliver them to the resort.
Some posters in the past have also shipped a package containing water and other supplies to themselves in care of the resort.