Amazon.com: CONNECTOR RIGHT ANGLE FOR SQUARE TUBE JOINT 2 WAY PIPE PROFILE PLASTIC 20MM 25MM 30MM 20x20 25x25 30x30 (A=20 x L=36 x L1=36 x t=1.5) : Industrial & Scientific
8 Sets 1 Inch 180 Degree Structural Pipe Connector Metal Joint Chain Link Fence Clamp, 3 Way Cross Metal Joint Stainless Steel 201 Corner Fitting End Rails Clamps, Thickness 1.8mm - Amazon.com
10 Pack Square 2 Way Nylon Tube Connector, Tube Fixing, Tube Joint, Heavy Duty | eBay