The ResMed AirFit N10 and N10 replacement headgear strap can be purchased individually so when you need to replace the strap but not the entire mask this gives you the option to save. 

Headgear straps will stretch over the course of time from tossing and turning and oils built up from your hair. If you notice that you must constantly tighten your mask it may be a sign that you need a new head strap.