1962 Triumph 5TA Distributor

I have a 1962 Triumph 5TA which has a distributor, the issue I have is that when I ride the bike there is oil coming out at the base of the distributor around the crankcase. There is an o-ring on the outside of the housing I have tried to source a new o-ring but the ones that I have purchased are always too small or too large. Problem with the larger o-ring is that the shaft will not slide into the engine casing.
Anyone have a part number for this o-ring or know where I could source the correct o-ring or has anyone had any issues with this paticular bike leaking oil from this area.
Any information would be greatly appreciated.

 

Thanks guys for the information I will check those shops hopefully they will have what I require.