I am looking for audio system for my car with woofer , so where can I buy or assemble in my car?

Also you will need to include amplification since most home subwoofers are powered and not powered by the receiver. In car audio you are working with a very small space providing a lot of room gain. This makes it possible to tune the woofer at a higher frequency than would be required for home theatre and still get low bass from the room gain.

If you put that same woofer in a theatre instead of a car you loose a lot of this and suddenly all the super low bass is gone. You shouldn't think in terms of how much power a subwoofer is, the power you are considering is input power so it would be a bit like buying a car based on gas consumption with the highest consumption being your goal. You should think it terms of what the subwoofers output capabilities are and ideally find one that can produce the desired output on the least amount of power.

Car woofers are typically operating in a very small box. To do this they use a very heavy cone with a very stiff suspension. The problem is that moving this heavy stiff suspension cone in a small box requires a lot of power to attain a given output.

A theatre woofer typically has a lighter cone with a loose suspension and is mounted in a larger box. This makes it require less power to attain the same output. In my personally opinion even typical theatre woofers use far too heavy cones with far too stiff suspension.

This all comes in due to marketing, one people don't want the box to be too big and also they like to see big numbers on the power...because they don't understand the numbers and don't realize lower power requirement is better. If you want a good theatre woofer and don't want to spend a fortune then you are a perfect candidate for DIY. You just need to build your own woofer.

If you have space for it IB (Infinite Baffle) will produce the 2nd best bass. Dipole (Open Baffle) produces the best bass but requires multiple high excursion woofers. Large sealed box produces the 3rd best quality bass and may be the best compromise for you.

Sealed is the easiest to design. Ported gets you a little more bass for less but at some sacrifice in quality. Instead of using a port to utilize the back wave of a single woofer, stay with a sealed box and use two woofers to get the same output.

It cost more and might be larger but it's better quality. The place to go is either Madisound or Parts Express. They have all sorts of woofers and even some easy to assemble kits.

If this is your first project then I would suggest a kit.