It was honestly just dumb luck. I had heard of these previously from a friend who had some in the Philippines. I would say, really, I do know nothing about nuts, relatively speaking :)

These nuts are indeed not really popular but I found them in a regular supermarket in Germany a couple of years ago. As someone mentioned they're typically almost impossible to crack, so each nut already had a small canal sawed into the side and the box came with a metal lever that exactly fits into said canal. With that preparation it was quite easy to open them.

You can also buy them online prepared that way. E.g. here: https://mypilinut.com/de/products/noix-de-pili-en-coque-seau-xxl-2900g (I don't know this shop, it was just the first result on Google, so no clue if it's a fair price).