Nilfisk and Tellus GM80 commercial vacuum cleaner heavy duty trolley rear wheel hub cap to lock the wheels on. See them on the bottom right of the photo. The drawing is a sketch of the trolley. The price is for one. There are two on a complete trolley.
As it's light, maximise the saving on postage and buy some of the related parts below, such as dustbags and the filters or a new floor tool.
At the moment, standard postage within Australia is capped at a maximum of $20!