Get files from opentopography.org or elsewhere and convert them to RAW files for Unity to use for terrain heightmaps
You'll need node and npm and git
>git clone github.com/fenwick67/unityterrainfromtiff.git
>cd unityterrainfromtiff
>npm install
>node convert.js inputfile.tiff outputfile.raw
Might register in NPM if anybody wants it.
If your file is under 4097x4097 and isn't exactly (2^n)+1 px wide and high, Unity will produce artifacts at the edges of your terrain. This is because Unity internally uses heightmaps that are (2^n)+1 square.
The image will get cropped to 4097x4097 (from the top left) if it's bigger than that. This is the biggest that Unity supports.