You can build one for cheap and easy.
Either 3/4" or 5/8" MDF wood.
3/4" is fine.
Buy the sub, then find out the recommended enclosure size. This may help.
http://www.rockfordfosgate.com/rftech/box_wizard.asp
Also, don't use screws unless you pilot hole them, even then, a nail gun of heavy duty staple gun works best, along with some glue. You need both.
Then they will let you know how big the hole needs to be. It will be slightly smaller than the actually sub size.
This can be cut using a jigsaw, just be steady.
You don't need to cover the box with anything, just the plain wood and a sheet can cost as cheap as like 20$.