Pretty much no manufacturer is going to sell you parts besides maybe replacement feet. But the only things that fail on mice are all jellybean components.

Left mouse button fails? Buy another from mouser. Middle button fails? Digikey. Side button? Some other components selling company.

Outside of those super light mice there’s nothing special about any of them other than the exact layout, and the case. And the cheaper the mouse usually the simpler they are on the inside, and the easier it is to solder. Most PCBs will be single sided with through hole components.