Do you mean the breather / cap on the rocker cover that has a pipe to the air filter coming from it? If so the seal can be had from Paddocks but it probably still won’t seal. I ended up removing the seal, wrapping tape around the area on the cap where the seal sits to increase its diameter, then refitting the seal to the cap. This had the effect of increasing the overall diameter of the seal thus making it a tighter fit in the rocker cover.
Hope this makes sense and helps
Rich