The reason for low profile subs is so that I have enough space to move my belongings in the summer.
OK, so there are 3 options:
1. Get shallow subs and install them under front seats. I thought about doing it, but every audio shop I talked to said it is not a good idea, since I will sacrifice sound quality.
2. Get a custom made box for the spare tire (it works really well, many people have it). You can also install your amp over there.
3. Get a regular 10"-12" box and just disconnect it when you decide to move (you can put it up front of your car or just leave it at your new place before you pick up your gear from your old place). iM is super tiny for moving. When I was a student I had a Honda Element and everything would fit into that car. You can also get a nice single 8" or 10" box, there is really no need to go with dual subs unless you are totally into bass heavy music.
If you just absolutely want to go with underseat subs, I would research pre-assembled small subs. Like Alpine, Kicker or Focal. Crutchfield is probably the best place for that, but their prices are almost always the highest. So I would research on Crutchfield, but buy on Amazon.
Powered Subwoofers at Crutchfield.com
If you just adding a sub, I would go with an "active subwoofer". That way you don't have to install a separate amp.