Paweł Łyczkowski

Pawel has a fantastic set of decals, the Stencil Kit - Industrial on Gumroad.
The download includes a folder for individual, transparent pngs, properly named for DECALmachine and ready to be batch decaled.

Antoine Collignon

Antoine has an excellent, colorful, high resolution decal set, called 40 Sci-Fi decals 2017 on Gumroad.
At this point, you still need to rename the images yourself, according to the naming rules to make the Batch Decal Creation work properly.