depends on how much you want to spend. By the sounds of it, you should upgrade to a JL stealthbox. It's two 8 inch subs that replace your rear speakers and fire directly into the cabin. Its not super huge bass, but its pretty good. It also looks great in your trunk. For that setup you can expect to pay $600 after all is said and done, possibly more.
If you don't mind taking up a little more trunk space and want to keep your rear speakers, try a pre-fab box with a 10 or 12 inch sub. If you go used, you can save quite a bit. An amp is going to run you quite a bit, around $200-$400. I am selling my old sub setup for pretty cheap, but you might be able to find cheaper depending on the quality of sub you want.
In short, there are PLENTY of choices out there. First, figure out your budget and what kind of look you want to achieve. Go from there and look into your options. Install won't be more than 2 hours work and labor is usually $30-$60 an hour depending on where you go. Feel free to pm me if you have any more questions.