This is a Holmes Air Purifier model HAP115Z. They are something less than 20 bucks at Wal Mart.
They charge like 7 dollars for the filter alone but you can buy a cut to fit filter with charcoal for about 12 dollars and make a LOT.
You can reuse the plastic grate that comes with the cut to fit filter so it only takes a few minutes to cut a new one.
Then it just sits flat in the back of the filter.
Cheap and Easy!
I've gotten over $200 in cash back from them at places I would have shopped anyway! Ebay, Staples, more! I put it at the top because they ROCK. Basically, you get a kickback from Ebates when buy.com or whoever pays them. Win-win. If you like throwing money away, don't use them!