I've bought my replacement from
http://www.easternseals.co.uk/ and it was a
60 x 100 x 10 R23 NBR OIL SEAL
. That's 60mm inner diameter, 100mm outer. They put a stainless spring in for me and charged me only £5 plus VAT and shipping.
You have to drop the rudder to replace it. On my boat, this was more difficult than it should be, due to very poor alignment of the rudder tube with the post. From the wear and distortion on your seal, it looks like it might be the same situation on yours too. I've slathered the seal in silicone grease before installation to lubricate and protect it from water.
Failure of the seal is not critical, it's fairly high above the waterline and only keeps some minor splash out.
Alternatively you can replace the seal, or both the seal and the rubber part of the tube with one of Jefa's neoprene gaiters:
http://www.jefa.com/products/sealing/sealing.htm