This Manfrotto R276,02 Rubber Foot is included with select tripods and monopods and is sold here as a spare or replacement part.
I lost one rubber foot for my Nikon tripod. The tripod was so old that the silver label listing the model number was rubbed blank. This replacement was the right shape and looked like it would fit, so I ordered it even though I couldn't find the dimensions listed anywhere. The replacement fit perfectly.
Had heard about using with a Manfrotto selfie monopod and Pixi mini tripod to use with the 360 camera. Works great and allows me to have an inexpensive sturdy setup. As well as, keeping the mini tripod and selfie separate when needed. All in a small setup.