To me, you don't need two plates - just one. They serve the same purpose a a belleville washer - that is, just provide additional tension to keep the stack tight.

PBR probably sells the pieces, but I don't know that for sure.

As long as there is at least one tension plate on the switch stack, you should be OK. If you have some stacks with two plates, you could "borrow" one of those two and move it to a stack that is missing the plate. The important thing is to make sure the screws are down tight.

The grommet cushions on a relay plate are there to help prevent the relay from being mechanically actuated by the vibration of the coils firing on the machine. If a relay has a cushion mounting, you should preserve it if you can. If you don't have replacement grommets, you can attach the relay mount without a cushion, but it's better if you don't. An o-ring is kind of thin, but it would be better than nothing. Grommets should not be all that difficult to find online if you don't have a local store that carries them.

I have personally tried mounting a cushioned relay without the grommets in a game I was rebuilding, and have had the relay get triggered during game play from vibration. So the grommets are important, and now I always make sure they are there if that is how the factory did it.

- TimMe

The coin relay and start relay typically are mounted on rubber grommets. This is because "kids" would kick the bottom of the machine to try to get free credits from vibrating said relays.