You get that o-ring in the carb rebuild kits you can buy (NRP Carbs do the kits).
I'm sure I have some used (but good) ones from my YZF (but I would think it is the same size).
I can post you one if you want to give it a try?
Allens performance can supply them separately. I've bought one before for a few pence
The link is on the left hand side of the main site front page Www.exup1000.co.uk
Thanks Stig, I haven't seen your answer. I've ordered a pack of 5 from "simply beargins", or something like that. I had to order the steering damper seals anyway so it came all in one pack. Now let see if it's the right size...