If you want to replace a leather washer with a high-quality plastic one, then Jim Maccari sells some which might fit your rifle and they are inexpensive.
http://www.airguns.citymax.com/page/page/251485.htm
Alternatively, T.R.Robb makes PTFE ones which may or may not be an improvement on the original.
http://www.trobb.f9.co.uk/