Supposedly, you can find those things cheaply on cars at a junk yard.
I wonder, though, would it be possible to tap into the rear speaker wires for the signal and connect them to a powered 12 inch sealed subwoofer? Or say an external amplifier? I have right now a 300 RMS watt subwoofer in a 1.15cu ft hatchback style sealed box (conveniently just tall enough to be pushed back all the way to the back seat in the trunk and virtually not take any space) stuffed with polyfill running off a 250watt amplifier channel. After tweaking eq, crossovers, and gains, it sounds fantastic. Really tight, loud, yet spacious and soft on ears. I can't imagine going back to a lesser subwoofer solution. And that empty oval hole is great for letting all that bass into cabin in full force. But it's a lot more expensive of course.