OP are you still with us ?
There are a couple of different Ford GT blues. A light powderpuff blue that goes on the early versions and a mid blue, which are you looking for ? Which kit is this, they often list a suggested paint to use ? Are you inclined to use acrylics or will you use solvent paints ?
In solvent paints Tamiya Mica Blue TS-50 I believe is the number for their lacquer, covers the mid blue. Zero Paints lists the light blue under Ford GT( but I don't see the name Shelby Blue) which I believe is also lacquer. Scale Finishes will mix any paint you want in lacquer or enamel.