I made my own, its really easy if you have a table saw or circular saw. I chose a sealed enclosure for cleaner bass, so the math was simple.... find the required airspace for your subs and build a box to match it.
It costs me maybe $40 to build my box..... MDF is cheap. There are a bunch of videos on youtube that can show you how to build them if you're not sure. Many videos on building fiberglass boxes too if you'll need to go down that road.
This is the box I built for my 07 DC. It holds 2 10"s and sits under the rear passenger seat.