There are two pieces to this button: the button cover and the backing which holds the neopixel and switch. 

I've uploaded a plain version of my button cover as well as a version with an "om" symbol etched into the top.  However, you don't need to use the 3d printed button -- jewels, bakelite, plastic, or even coated metal will work just fine.  Play with some different ideas to discover what you like.

If you don't have a 3d printer you can order the buttons from Shapeways (button front (Om) or button front (smooth) and button back), or you can make your own out of PVC tubing or posterboard.  

The backing is just the right size to fit the electronic components inside, and the wire holes will perfectly accommodate 30awg wire.  There are a few extra holes in the base for sewing the button to your project.

Print the pieces you'd like in ABS or PLA.  I found that white ABS had a nice diffusion to it, but ended up using glow in the dark PLA for my final buttons.  This stuff has a lovely translucence to it, making the buttons look prettier when the lights are off.. and of course, there's the added bonus that they glow in the dark quite vigorously after the buttons have been turned on for even a minute or two.