I suggest you build one your self.

Calculate the volume of your grow room, and have your fan try and exchange that every 5 minutes.
That's a rule of thumb... not exact science.

Anyways, you can get activated carbon at the pet shop... probably cheaper if you search around.

Build your self a box...
Put some window screens' material at the bottom...
Make a layer of activated carbon.

Attach fan to top side... pull air through the bottom.

That's the basic primer.

Helps?