I second the JL Audio 6W0 reply as the most reasonable option. They work well in small sealed or ported enclosures. Focal also makes a very small 5 3/8" subwoofer that is supposed to be excellent, but you will pay much more. It's part of their Utopia line, model 5WS.
I run 4 6W0's in a custom self-built fiberglass/MDF enclosure. More cone area than a 12" sub in a stealth package that could take 500 watts easily and still be invisible.
JL's website has lots of specs on their subwoofers including expected frequency responses for different enclosure designs and volumes. Do a search at carsound.com for more info. Mannville Smith, the owner of JL Audio posts there as well as a couple of JL's Technicians.
Edit: I found Focal 5WS at woofersetc.com for $116. Much less than what their recommended retail is!!! Could be more of an option than I thought, but do your homework. I think they need more airspace than the 6W0.