Hillman 3/8 in. I.D. x 3/4 in. O.D. x 3/8 in. W Shaft Collar 58329 - The Home Depot
Climax 1 in. Bore Zinc-Plated Mild Steel Set Screw Collar C-100 - The Home Depot
Amazon.com : Dog-Thing by Talents10 - Dog Training Prong Collar (Large, 3.5mm Thickness), Metal Collar for Large Dog Obedience Training. Better Than a Choke Chain or Other Dog Collars for Dog Walking :