There are many AR-15 flash hiders/suppressor on the market. If you are building your own AR-15 then you will need to buy a good quality flash suppressor like the Tactical Intent A2 MIL-SPEC suppressor. It comes with the crush washer and is the typical military issue suppressor. The Tactical Intent suppressor is made in the USA and parkerized. The thread is 1/2-28 and fits most M4 barrel configurations.
Read the reviews or buy the Tactical Intent 5.56 AR-15 flash suppressor here…TACTICAL INTENT .223/5.56 Flash Hider AR15, A2 style, MIL SPEC, INCLUDING Crush Washer