Totally depends on what you're looking for. Do you want a discrete add-on just to add some extra base? In that case you can find many all-in-one slim subwoofers designed to fit under a seat. I had the kicker hideaway 8 in and it fit perfectly under the passenger seat and provided some extra low end to help the stock system sound more full.
Now if you want to go balls out the sky is the limit. I've swapped all the speakers have a custom box with two 10 in JL subs in the trunk plus two amplifiers, one for speakers and one for subwoofers under the driver and passenger seats respectively.