My power button fell off. Missing a spacer?

I have no experience with this specific device. However most buttons on portable electronics are built the same way with a rigid spacer set directly above the center of the button's dome. The spacer is usually glued to the plastic or metal key which your finger touches.

These spacers are often less than 1mm thick, so finding a suitable replacement material, then cutting that material down to a 1-2mm diameter disk can be difficult.

...So don't replace the spacer with a one-layer material! I routinely replace spacers missing from cell phone power /home/volume buttons using many layers of tape. Polyamide tape, better known as the 3M brand Kapton, ranges from 0.1mm to less than half that thickness depending on the product you buy. The tape carrier material and adhesive do not compress, meaning if you taped hundreds of layers flat, one on top of the other, you could stand on this block without it deforming. Yet a 0.01mm tape is very easy to cut to shape in individual layers. You can fix your issue on just about any button with a roll of polyamide tape, sharp scissors, and fine tweezers.

Lets say hypothtically, you are missing a steel spacer that is 2mm x 2mm x 0.9mm. I would take a roll of 0.1mm polyamide and cut a 2mmx2mm square. Using tweezers I would stick the square in place of my spacer. I would then continue cutting 2mm squares until my spacer was 8 layers thick. Then I would cut a 2mm x 10-15mm strip and stick it on top of my spacer, providing some support against lateral shearing. You now have a perfectly sized replacement for your little steel disc.

Now practically speaking, you would never be able to measure the thickness of a part you are missing. So keeping the above example in mind, start with several layers and then add more until you find the correct thickness. In ten years have probably done this 500 times across 50 different phone models.