Our Post office carries them, and they sell them for $8.50 thats teh Horizon, with Filters already installed. there are a few others out there but the shipping is often about the same as teh price of the boxes unless you buy at least 50.. I wish you luck.
There are several PO's across the country that keep them, especially those associated to airports where shipments of birds are common. Check with your local PO, we drive an extra hour to teh airport PO and ship from there since they normally have the boxes in stock and it cuts a full day off of delivery times.