To be honest any good branded cone air filter you can get to fit propperly will do the job, they all work on the princible that they filter the air unrestricted, green, k&n, pipercross, blitz, all good brands, avoid the cheap chav tastic china copy stuff.
Halfords sell off k&n 57i kits cheap in the ripspeed bargin sections but your have to travel to a few to find one