Need a standalone subwoofer for my PC.

I've got good speakers connected to my computer, and I would like to connect at subwoofer to improve sound.

I search around, but all subwoofers are >10k .
Any wub for like <5k?

Can I connect a car sub?, and what amplifier will I need?
Also, why are car subs avaliable and cheap, whereas pc/home are nowhere to be found?

Please recommend.