Well, you either generate it (looks like shit => do not do this) or create it by hand.

Actually you can also use some kind of combination of both methods (I usually do this) but you'll still have to do a lot of hand panting and modifying.
If the heightmap isn't accurate or doesn't match to the shape of the texture you're trying to paint/generate it's useless, so reconsider if you really want to use this.

And btw.: Normalmaps are usually much better when theyre generated from high quality heightmaps.