In case you haven't found what you are looking for here is an option. U-Haul has some 22 x 22 x 12 boxes for $3.95.
Here is the link.
http://www.uhaul.com/MovingSupplies/...wh=22x22%20x12
They also have an Electronic box that is 20 x 20 x 12 but that is $5.30, probably heavier.
They do have a mirror/picture block that is only 4" thick but that is much larger at 37 x 27 inches for $4.25.