JL audio makes the 6W0 6.5" sub which fits in the factory enclosure. It's a tad expensive at $80-100, but you can get them online for less. Otherwise, you can find various cheap 8 in. subs and modify the enclosue slightly for them to fit, or even better, get the JL 8W0 8" which isn't much more than the 6W0, and throw that in.