Wondering where I can buy these? I think the correct name is "Radial Shaft Seal". The one on the left in the photo is for MTD/Rover, the one on the right is Victa. They go behind the wheel on the axle to keep out dust. Over time they wear out and sometimes get damaged if a bearing gets rusted to the axle. The Victa ones go brittle with age. The Victa one has the same axle diameter as Rover/MTD but has a smaller OD. I am finding that the ones on MTD wheels are a mm or two too wide for older model Rovers (the centre bit sticks out too far), so I've had to grind a bit off. They sometimes come with new wheels, but am having trouble sourcing them as a standalone product.
For some reason I didn't give cup washers a thought. That's a great idea! The only 3D printing place near me usually wants over 100 bucks to do the design/setup but then the part is around $20. We need a few others to start up to go in competition with him.