Go to your friendly local bearing and seal place and buy a 26x7x16 seal.
However, if you do not know how to remove the old seal (and put a new one in for that matter) stop at this point unless you have a knowledgeable friend who knows how and can show you.
It's not hard, in fact you will probably be OK if you follow Snowbum's essay on the topic (bonus points if you can stay awake to the end bye-the-way). There is a bush in there, I've never had to change one, but while the seal is out you should test the bush for movement - putting a new seal in after a worn bush will mean that you waste the couple of dollars for the seal and 30 minutes of your time.
Oh, and one last thing, after removing the fuel tank (and your battery if it is not a fully sealed or Gel type) lay the bike on the RHS with the rocker resting on a lump of 4*4 with an old sock on top of it - otherwsie you are going to lose about 300cc of smelly 90 weight oil when you pull the shifter shaft. on the other hand, this might be a good time to give your bike a "getting to know you" gearbox, shaft and final drive oil change (you need 2 litres of EP hypoid oil and a thing called a "sludge pump."
Sorry, a second last thing - before you pull the drain bolts on the gearbox, shaft and final drive, try loosening the filler bolts first....Just saying cause it is a crapload easier to solve problems on the filler side before you drain all the oil out.
1981 R65 you say? Interesting that it has the earlier "bent coathanger" shift link (I think they changed 1980 year model but someone will correct me if I am wrong on this).
Overheating your engine is incapable of transferring enough heat to the gearbox to do it any harm - I suspect your leak is more to do with the learning process your left foot is going through getting used to airhead gearbox idiosyncrasies.
