I have some options in aluminium 12mm washers.
They aren't the same thickness as the original Bullseye ones (I never managed to find anything exact match), but can be stacked to make up to within 0.5mm of the original heights.
As follows:
13 x 19.5 x 2.0 stainless steel
12.4 x 17.8 x 1.5 aluminium
12.2 x 20 x 1.5 aluminium
12.2 x 20 x 2.0 aluminium
Here's the Bullseye axle crib sheet that @yakboy sent me in the first place.