from some factory's cutting holes in huge steel plates. 20c a lb or so. 3/16" thick is ok for .22lr, 1/4" for 9mm, 3/8" for .45. Thinner means dents in the metal from bullet impact, heavier means you can barely tell if you hit it. Drill two holes near the "top" of each disk, hang by a wire is an easy way to have 'reactive" targets, but the bullet splash or direct hits often cuts the wire. I always just welded chain "half links" on the top rear of each plate, which helps in this regard.