R.J.
In a PINCH... use an empty soda/water bottle. Melt (or cut)holes to fit the tubing at the side near the bottom and exhale holes (if yours uses them) up at the top where you breathe in through the drink opening. If you need a tight seal on the ingoing tube, secure it with a little bit of painters tape or duct tape.
It'll work until you can get a replacement part.
((Same works as a spacer for an inhaler in a bind, btw... but you only need a hole to fit the inhaler "mouth" piece))