3/4 x 24?
If you're talking about a crush washer for a .30cal, then Brownells has them in stock
www.brownells.com
If you can use a standard aluminum crush washer for automotive use here's some.
Earl's AN 901 Aluminum Crush Washers -08 AN, 3/4" I.D. - (5 Pack) : 177008ERLP
Here ya go---Anybody know where I can find a crush washer for a 3/4x24 threaded muzzle? I'm not having any luck.
Shim kit or peel washers. Couple options that popped up on google for 3/4-24.
Shim Kit - For M110 3/4-24
https://kahntrol.com/threaded-muzzle-brakes/
https://thunderbeastarms.com/products/accessories