There are three main types:
1) tree bark, like this one at our BRC library in Kolkata:
2) palm or tal leaf, which is usually the oldest and rarest kind. This one was donated recently:
This was part of a batch of 20 that were found floating around in the big Mayapur flood of 2000 and were rescued by a local man. Most were ruined but a few survived and we recorded them for posterity.
3) a variety of different types of paper, such as this cotton based sheet at Halisahar Caitanya Doba mandir: