Local hydro shops. They will probably run you $120. If not that online would be my next guess, or perhaps a homemade one?
Here is a step by step with pictures. i would reccomend bookmarking it http://forum.grasscity.com/grow-roo...cheap-easy-efficient-diy-carbon-scrubber.html
Wormsway.com CAN 2600 carbon filter (make sure you buy the flange as well) Well under $90 shipped (make sure you buy the flange as well)and works better than homemade. I made one and am a pretty handy guy, but it blew elephant penis. Buy one, it will serve you better, i promise. I guess the 2600 may not suit your application, so make sure you get one to match your cab size and fan. Mine is in a 3FTx4.5FTx7FT tall space and works tits.