You can find the H3 buffers at Midway (I've got Colt and AR-Stoner), or you can buy the tungsten weights, and adjust your standard carbine buffer. I've done it both ways, very easy.

https://www.midwayusa.com/s?userSearchQuery=H3+buffer&userItemsPerPage=48

https://pur-tungsten.com/product/tungsten-recoil-buffer-weights/