Gunsmithing where to find 3/4 in crush washer?

Time the brake no washer needed

I don't understand what you mean by 'time the brake,' if it's not a directional brake, it doesn't need to be timed, if it's a directional brake, it needs to be timed, washer or not.

Timing the brake would require removing metal from the brake, or the shoulder of the barrel, or flat washers.

Better option is to buy a regular washer and sand it down to time the brake. Also, I've read that crush washers can force the brake to be slightly off-axis, not sure of the truth of that though.