The prescription thing is probably location specific, but AFAIK you can just buy them online. I have a few spares I bought online from back when I frequently slept in different places.

Masks straps could probably be fitted with an accelerometer to detect positioning. If they can make heated tubing, this isn't such a stretch. Might also be useful in correctly flagging events: my machine often incorrectly marks position switches as periods of hypopnea.