The type of installation you are talking about are normally custom installs dealing with fiberglass. Any reputable shop should be able to accomplish this style of install. If you are looking to just add a little bass, a simple 10" or 12" woofer designed for a sealed box, and an amp that matches the sub would suffice.
Again, go talk to a reputable shop, and look at their install book. Make sure that if the book is full of fiberglass installs, that the installer that performed them actually still works there. Normally a shop will keep the same book for years, and installers move around.