Leather is the perfect material for the application. Cork does not rebound as well when compressed and therefore does does not have the same "anti bolt loosening" properties as leather. Cork also dries, rots, and crumbles many times more quickly than leather. Cork shares this property with rubber, BTW, which is why you can find 50 year old bikes with original leather washers, but not cork or rubber ones.
In addition to dedhed's idea, steel punches are available in a variety of diameters and will last essentially forever. I am lazy, myself, and simply buy washers from Velo Orange. I did make the large washer for between the fork crown and front fender myself, using an old leather mudflap and a pair of scissors.