I built a terrain height map completely from free data. https://codetiger.in/earth/

I will look into this, thanks! In the meantime I'm going to implement a fallback to 2D tiles which has much higher quota limits. Try to stop the bleeding first.

It's somewhat old, but NASA has its SRTM height-map dataset. I wrote a project for reading this in Python many moons ago, in case it is of help to anyone:

https://github.com/adamcharnock/python-srtm

I originally used it for doing line-of-sight calculations for a wireless ISP I started.

Europe LiDAR data: https://sonny.4lima.de/