Hillman 2-Pack 1/2-in x 1/2-in Black Plastic End Caps in the End Caps & Screw Protectors department at Lowes.com
Pack of 4) 1 1/2" x 2 1/2" - 12 - 16 Ga - Snap In Rectangular Cap Plug 1 1/2 x 2 1/2 X 17/32 Inches | Fence Post Pipe Tube Insert - Furniture End Cap - Fitness Eqpt Finishing Caps. - SB Distribution Ltd.
Standard Galvanized Post Cap 2x4 (1-1/2" x 3-1/2") - 2 Pack - - Amazon.com
2x4 Stud End Cap by skear - Thingiverse
1 1/2" x 3 1/2" Flat Galvanized Steel Post Cap for Wood 2x4 -(Fits Actual 1 1/2" x 3 1/2" OD Wood) | Square Fence Fittings