I made a short (4 mile) hike to work today and wore the bamboo Injinji liners. After that, I think they are closer to cotton.
The walk in was fine, but after an hour in my office, my toes were feeling cold. The liners were still damp. I took them off and immediately felt warmer. They were still a bit damp after two hours. I've had better luck with Injinji's wool and Coolmax socks.
They might be good for warm dry weather, but I won't use them as general purpose liners.
As for the question of where to find them, REI has some on their site. I got mine from ZombieRunner.