Where can I buy Bass Woofer for laptop?

Hello, I have a lenovo z580 laptop, in which I wanted to attach a Bass Woofer (idk what is it called ).
I mean it has a really good sound but no bass. So, basically I want a speaker which only has bass and the rest of the audio part should be played from my laptop's speaker. Also, please make sure that the speaker should be a little portable.
thank you

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