Hello Mattijs, sorry I didn't see your question until now. That instrument is sometimes called a kalimba and some versions of it called a mbira. It's a common instrument in some cultures located in Africa. In English it's sometimes called a "thumb piano." It is, indeed, relaxing and a fun thing to have. 

You can find them to purchase online in lots of places. They're not expensive. 

And you can also make them (they were originally folk instruments made by hand with wood and/or gourd resonators.

Here're a couple of DIY links:
How to Make a Kalimba (Thumb piano) / DIY 2018
Trash Kalimba Musical Instrument