The coil spring is an off the shelf spring. Here’s the link to the bit I’m using for 8mm dominos. I’ve ordered bits to use for 5,6, and 10mm. The 8mm happened to the the only one delivered when I finished the project.
https://www.amazon.com/dp/B00004T7BC?psc=1&ref=ppx_pop_dt_b_asin_title