Have a look at a pack of 250+ Hard-Surface Alpha Maps by Jonas Roscinas that features more than 250 alphas made for hard-surface model detailing, texturing and sculpting. 

The set also includes handles, bolts, primitives, vents, seams, and other useful stuff. All the files can be used for commercial projects. 

Alpha Maps are 2048×2048 and 16bit of depth, in .JPG, .PSD, and .PSD formats.

Best to use with:

• ZBrush
• Mari
• Substance Painter
• Substance Designer
• Blender
• Quixel
• 3Dcoat
• 4Dcinema
• Mudbox

You can learn more here.

Get the pack for $9+