Go to your LBS
DCepp said:
If you ask around at some LBS you will usually find that they have a bag of extras left over from other helmets..
You'll either pay a little bit or nothing at all for them.
Easier still, as the other poster said, Giro will send you some for free, but they may only fit a specific helmet.
I suppose you could also cut up some foam or sponges or something and then figure out some way to make it stick.
You know, a shoe repair shop may have something that they put on the tongues of shoes...
If all that fails, you could buy some bubble pack and stick it around the inside...but it might get sweaty without venting.......And if you crashed on your head it might make such a popping noise, you could be left deaf.....
