I found one for mine using a rubber "O" ring that I found online. The chances of finding the correct size/thickness at a hardware or antique sale are next to none... Measure the diameter of the metal wheel and then search for one that is fairly thick, since most o rings are meant for sealing against leakage of some sort and are usually thin... I may have found mine on Amazon, but not sure. You might also search for reproduction toy tires of the correct size...
O rings for hose ends may work, if they are the correct diameter...
https://www.amazon.co.uk/s/ref=nb_sb_ss ... 5PUIBORY8I