Ask Mansugi. He makes both white X-wing pilot flak vests and grey vests for people dressing up as Baron Fel.
By the way, the flak vest that I got when I bought a Costume Base jumpsuit was slightly purple in color, and it did not go away with any type of bleach that I had tried.
The real thing was made in a coarse synthetic fabric such as nylon or polyester. I think Mansugi makes them in nylon.
There are also a few other people in the Rebel Legion who make flak vests for X-wing pilots. Over there, Mansugi calls himself Mr Bojangles.
The usual price for a well-made flak vest is USD 100 -- they are labor-intensive to make, with all the ribs.
I have made four myself. I don't make them for sale, but am willing to share freely all that I have researched about this garment in case you want to make one yourself. There are a couple of threads on the forum with detailed info about the real thing, including the original British high-altitude flight suit.