What is the best way of finding D3D9 resource leaks on a Windows 10 machine nowadays?

I must add that the leak is not easily reproducible also.

I find it more and more difficult to work with legacy D3D9 applications since the debug version no longer works on a modern Windows. Are there any new tools available that can help me, like nSight or GPU PerfStudio or such?

If you've got your own D3D wrapper, then whenever you create a resource, capture a call-stack and a user defined name and store it alongside the resource. When shutting down, print out the names and creation-call-stacks for any leaked, objects. That will at least tell you where it came from.

It also helps to know the most likely causes of leaks: getting a surface (either from a texture or from the backbuffer) but not releasing it afterwards, or failing to call the OnLostDevice method of any D3DX objects you've created.