I buy most of my parts from ebay, trick is to look for a seller who has any O series items, or any oil seals for sale and see if they have an ebay shop, then email them to see if they have this part.
Also the O series was found in montegos, mastros, to name but a few rover cars, not sure if the part would be same as these were front wheel drive cars. but it might be worth looking a rover owners clubs for parts there.
Good Luck
John