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I'll be getting the Spear of the Emperor Special Edition sometime either this month or next. It'll have a decal sheet for the Emperor's Spears chapter. However, as pointed out on other forums, the amount on the decal is not enough for a full 40k army.

I myself would like the option to build a full 40k army of the Emperor's Spears.

I've made decals myself before, but have never made a copy of a decal sheet. There's also third party options at least for the shoulder pads. So my question is, should I copy the decal sheet, and if so, how? Or should I go with some of the 3rd party options?

You could just scan the GW decal sheet, use photo editing software to remove the blue, then print it as usual.
Here’s one alternative, though it lacks skulls.
http://www.bolterandchainsword.com/files/file/322-emperors-spears-decal-sheet/

Ultimately it’s personal preference. I favor 3d shoulder pads, but I’ll use decals if it feels right, like on some vehicles.

https://www.chaptercustomizer.com/

I don't know if you're given permission to copy the decal sheet for your own use, but if you are (or you're shady) the cheapest option would be to scan what you have and print on decal paper.

GIMP is free (and better than Photoshop, IMO), if you go the scanning and printing route.

https://www.gimp.org/downloads/

Colour Select shortcut is Shift-O