Hey mate,
There are a few options for you.
If you have the five seater, there is a storage area under the forward floor.
I managed to fit two ten inch woofers under there with a custom box.
If you don't want to go that far then be aware that most of the under seat active woofers are fairly poxy.
There are a few good ones,
Hideaway: $450
Kickers one is $150wrms and quite good, however I prefer the one below.
optisound auto8 active: $350
Vibe make an active woofer with an 8" and is also ported. Roughly around $350 each. They sound the best out of the ones that I have heard so far.
TS-WX710A $499 (roughly)
Pioneer make a pretty darn good one as well, it isn't an underseat one, but it can be fitted into pretty much anywhere.
Expensive though.
Cheers
Evo