Apperently, Tang Band is offering a package of a 4" subwoofer driver and passive radiator. I can find the driver on Parts Express, but not the combo in the US.
Driver:
Package:
http://www.costruireaudio.com/en/pr15-a-subwoofer-bundle-w4-2089-pr15.html
Suggested AMP:
http://www.tb-speaker.com/products/ams141-150
Suggested enclosure:
http://www.tb-speaker.com/products/sugs-23
I guess you’d have to source the amp directly somehow and build the enclsure (they do have exact details on the product page), but I imagine this could be a decent little guy for a small set up.
Showcase claiming it can reach 33Hz (NOT ME, and don’t know that I would have gone with red…):