They are located in three places:
* On a table next to a cauldron: There is a pile of vials next to one of the cauldrons in the classroom.
* On the shelf behind Snape: There are some vials on the shelf behind Professor Snape.
* On the table in the back of the classroom: There are some vials on the table in the back of the classroom, near the window.
Remember that you can only use a limited number of vials for each potion you make, so make sure you have enough before you start brewing!