Exactly...
Dealing w/ what you have & not spending a dime, InsertNameHere, try wiring those subs in parallel w/ the amp bridged to maximize output. Hopefully, they're arranged in this manner. And, if that's not working out, you might want a bigger amp... or completely upgrade this area.

For cheap, affordable power, US Acoustics amplifiers come to mind. (Just tune gain appriopriately.)
For good bump/thud(s), may I suggest 2 10" subs? Just make sure that they can take a healthy amount of power. Or, you can do the 2 12" thing... your choice!

And, the only online custom sub enclosure I have seen come from sounddomain.com. Aside from that, you/a friend will have to hook you up w/ a custom sub enclosure.