Have a look at a pack of alphas for mech and sci-fi designs in ZBrush.
This set features custom 137 subd alphas you can use as SubTools.
“Don’t need to increase mesh density to add details. Work on different types of mesh, like a dynamesh model, decimated meshes and quad meshes.etc. Just take care to have smoothed surface or flatted properly. Once added on your surface, you can change, modify or even remove it later.”
Both the alphas and model can be used for commercial work. You can find personal ($20) and studio ($50) editions here.