I own a '98 SuperCab F-150 and I was wondering, what kind of subs would be good to use under the backseat, what amp should I use, and are there any websites that sell a box of this specific cab or would I have to build my own?

 

Supercrewsound makes pre-fab boxes, but it kinda limits your sub choices to what matches well with that box.

As for what subs and amp to use, it depends on what your going for and what kind of budget you have.

 

Check these out, they are shallow mount and I run them under the backseat of my 09. You might have to go to the 10" ones because they are even shallower.

http://www.sonicelectronix.com/item_...C-XW1200F.html

Alpine type S is another really good shallow mount sub. Neither are too pricey.
I have a custom box for mine.