Google your favourite online vendors or auction sites...
Your options are Stencils, and/or Paint Masks, PE or Vinyl:
J's Work Marking Mask for 1/35 WWII US Star for ShermanPPA5028 or Eduard 1/35 US Stars Medium paint mask # XT004 or many others
Or aftermarket rub-down transfers/decals:
Archer, or Peddinghaus, Xtradecal, Star Decal, or many others.
Alternatively, trace the yellow decal outline on Tamiya tape & spray your own. I think painting your vehicle OD, then paint the white star on top, I think that looks better.