ReliaBilt 2.5-in x 1.5-in x 2.5-in Black Steel Corner Brace in the Angles, Brackets & Braces department at Lowes.com
Amazon.com: 18pcs Stainless Steel Corner Braces - 40mmx40mm Heavy Duty Corner Steel Joint Right Angle L Bracket Brace Set with Screws for Shelves Furniture Wood Wall Hanging Fastener
Metal Sales 5.06-in x 126-in Unpainted Outside Corner Post Metal Siding Trim in the Metal Siding Trim department at Lowes.com
Stainless Supply | Stainless Steel #4 (Brushed)
Schluter Systems ECK-K Stainless Steel 2 in. x 8 ft. 2-1/2 in. Metal Corner Tile Edging Trim K50V2A/250 - The Home Depot