I created one when they first came out and now, I have totally forgotten where to go to get one.

You should be able to create them in the crystal room of the excavation area in the Sacred Lands.

Here is the main link to the place.

It's the area just next to the Oracle's tower near the top.

This'll bring you to where you actually go to make one.

https://subeta.net/explore/steele/prizes.php?crystal