Try an electronics store. It's just a subwoofer, just because it came from a car does not make it all that different than any other speaker. Go to electronics store an tell them you need an amp for a subwoofer that can handle I'm guessing 4 ohms, it may be 2 in which case you may have trouble finding a home amplifier to power it.
You'll be looking at something like this:
can probably find something cheaper, but that's just the first thing I found. You might be better off selling the sub and buy an actual home theater subwoofer for 2-300 bucks and end up happier with the set up.