I have one in my truck, and it sounds great. I actually didn't go with the MTX subs though. I went with a set of Rockford Fosgates. Be VERY carefull when specing out your subs, as the enclosure is very shallow.. There are not a lot of options out there that will fit in this small of an enclosure.
I also went with a different Alpine amp then the amp MTX offers. I only wanted their formed enclosure, nothing else!
The other option is a JL Audio Stealth box. This one replaces the center seat up front, and has 1 12" sub in it. Some like the MTX, some like the JL. I'm completely happy with my decision, and would add another if I ever got rid of this truck for any reason.
haven't seen one for sale.. Mine was custom built for 4 10's.. I didn't like the way the 10's sounded so I redid the top of the box for 2 shallow 12's. Sounds a lot better.