What are the replacement seals for the carb caps
https://www.kegland.com.au/carbonation-line-cleaning-cap.html
In my opinion the plastic carbonation caps are superior to the plastic ones as they use a wedge seal that pretty much never needs to be replaced.
If you do want to replace the carbonation cap seals then you can use this camlock washer:
https://www.kegland.com.au/5-pack-camlock-replacement-silicone-seal.html