Crutchfield has several different powered subs that should fit easily.
Link to powered subs, sorted by price. Several flat 8's that should work.
http://www.crutchfield.com/g_51000/P...rs.html?tp=114
I just did the stereo in my short step with a Kicker 8" sub under the seat next to a Kicker 400.4 amp (front two channels drive mids/tweets and rear is bridged mono into the sub.) Both fit under the seat well in my truck, but you would need to do some tape measure engineering to make sure yours would have sufficient space. Mine sounds awesome with 5 1/4" mounted in the door panels and the single sub.
