Try your dealer or bikebandit - or triumph performance USA actrually.
It is the gear selector shaft oil seal.
If you go to www.bikebandit.com
you can look the part up under your model bike on their parts list.
To replace it properly you should have that cover off, which will also require a new gasket for the clutch cover.
Maybe you can replacve it in situ - fiddly but probably doable.