It states at http://fallout.wikia.com/wiki/HalluciGen%2C_Inc. that there are 9. I only found 8. Can someone verify that and where it is? That would be great, because i need one right now...

I see you broke down the canisters before finding the quest which requires one.

I may be able to help, but please don't hold my memory against me. It's been a while.

There are two canisters which are easy to overlook, and it seems you found one, which is next to the box on the shelf of the upper storage unit. The other one is inside a device which fills them, near the back of the room. There are about 3 or 4 damaged canisters laying about, and the good one is near there. You should see a faint green glow coming from the machine.

The only other one I can think of that may be overlooked is the one sitting in the center of the room atop the chemistry tables. There's a bunch of junk on the tables, so it may be easier to overlook.

Finally, check down the stairs that lead out of the area. Sometimes, during fighting, one of the canisters may find itself down at the bottom.