Where can I get rubber sealing rings for McAlpine Shower Waste?

I have a McAlpine Shower waste trap fitted to my Daryl shower. The top of the waste trap is removable for cleaning. There is a 80mm dia rubber seal which has lost its elasticity and needs replacing. Daryl don't use this waste anymore and cannot provide a seal and McAlpine only deal with companies rather than individuals. Any idea where I can get a seal. It is rectangular in cross section.