I would say the subwoofer you want depends on where you are taking your setup. Since you have a pair of bookshelfs, it seems like you are just using this for music, although the receiver is capable of 5 channel output. On a budget, the sub you want to get for home theater is different from what you would want for music.
If you are using the system for movies, I would save up and get
this,
this, or
this.
If you are using this system to listen to music, bandphan's suggestions are good ones, there is also
the HSU STF-1.
If you want to listen to music and movies, I would get the HSU STF2, or maybe the HSU VTF 1 or VTF 2.
But really, any of the above mentioned subs can do either well respectively, it's just that movies have deeper frequency content than music so if you are just listening to tunes, you don't need to pay for the things subs do to get deep extension.
Subs are different to shop for than bookshelf speakers though, whatever bestbuy carries will be badly outmatched by online companies, especially at the same price point.