If you find the size it might be worth doing a group buy from
Polymax or similar.
If you're spending around £15 it gets much more viable, I had some O-rings for ZX6&10 oil coolers from them, I got 15 of each for the same price as 2 from a Kawa dealer. Viton is ideal for fuel related items though NBR would probably be absolutely fine on the choke plunger.
Note that O-rings are specified by the INTERNAL diameter and the material thickness.