1/2" Plastic Dowel End Cap, Black - Craftparts.com
3/4" Finial Dowel Cap, 1/4" Hole - Craftparts.com
Expansion Dowel Caps
Dowel Caps and Connectors - Plastic On Excel Dowel & Wood Products, LLC
Wood Dowel Caps 1-1/4 inch Diameter with 1/2 inch Hole, Pack of 10 Unfinished Dowel Rod Caps for 1/2 inch Dowel Rods, For Crafts and DIYers, by Woodpeckers - Walmart.com