One Industries sells 18" Carbon Fiber sheets that should work well for tank decals (very thick material).
You can use the carboard backing for a pattern. Trace the original bubbled decal with freezer paper. The CF material is dark enough to somewhat match up with the OEM decal . . . And only $15/sheet. I ordered 3 more sheets for the left side, front KTM decals, fork tubes, and rear swing arm.
I have used this material on my SM swingarm and fork tubes with very good results;