The Kao Lam vendors are relatively difficult to spot since they only have a very limited amount of Kao Lam with them and they usually carry it on the back of their bikes, in a small basket or the like. Really easy to overlook.
The vendors are driving all over Samui and it's to stop them. The price depends on the bamboo size but is typically around 40,-THB for a big one or 50,-THB for 3 small ones.
This is how they look like once you peel the bamboo off, this being a black rice variation Kao Lam. If you like sweet, they're really nice, although I prefer the white rice ones. If you see them around, try to stop a vendor and buy some, they're well worth trying out.