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You have also three good tools :
- NormalMap Generator https://github.com/Theverat/NormalmapGenerator/releases
An easy and very quick to create normal maps
in INPUT - you enter a diffuse/albedo
and
in OUTPUT - you can create a normal map DirectX or OpenGl, a specular map and a displacement map as you want.
- Materialize (https://boundingboxsoftware.com/materialize) which is more powerful
- Material Maker (https://www.materialmaker.org/) IMO which is more complicated to use.


Rotonut44 is right, through GIMP or Photshop it's not hard or bad to do.For future reference, and for others stumbling upon this topic: I did a short tutorial video for this in both Gimp and Photoshop a while back. It's really not that bad to do in either program.

Yep, invert the green channel in Photoshop or GIMP. Paint.net has plugins such as ModifyChannel or Curves+ that can do the operation (for Curves+ I added an InvertGreenChannel.xml to do the trick).