Google 'overdrive repair services'. There are a couple of UK companies who can provide the rear oil seal.
Note, buy a couple as they are tricky to drift into place and easily damaged (or they were in the case of the D type I just did, I think J type is similarly difficult) - you need carefully matched sockets to drift the outer ring of the seal into place.
Hi I have bought a new seal from ebay. Hope i can repair the oilleak when i got the new one. It was very difficult to take out the old seal without the correct tool. Maybe I have to make a tool to mount the new.