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I'm not into 40K, but what I'm looking for is probably included in one of the many figure/vehicle sets that I'm totally unfamiliar with: a star-shaped waterslide decal.
Preferably white and five pointed like this

You might be better off looking at decals for WW2 stuff - Allied vehicles made extensive use of white star insignia, and you should be able to find them in plenty of different sizes.

Oh, never thought of that. Good tip!

Like this? https://www.historicalboardgaming.com/USA-007-Decal-United-States-Star-White-_p_3475.html

How big do you want them?

The obvious choice is USA armour, which used white stars on their tanks and whatnot. So for example a decal sheet designed for Sherman tanks. They'd often have white circles around them, but you can just cut the star out to exclude the circle if you want, and many of them will come without the circles.

US aircraft from 1942 to 1943 had a white star on a blue roundel, so if you want the blue roundel/white star combo then WW2 US aircraft from that era should have them.

If you want a red star, anything Soviet would be the obvious choice. Aircraft decal sheets in particular will have lots of red stars, some with white outlines, some with black outlines, some just the red star by itself.