Ok, the only thing that you are goign to find are a pair of kicker 6x9" woofers. I don't know if you can still find them, but they are designed for free-air installation. So, basically, replace them with your current 6x9's and you are good. BUT you are going to have to use the proper amplifier and crossover to make them effective - this isn't a simple drop in replacement.

What may make you happier, is to simply remove the factory rears, and use a powered subwoofer like an infinity basslink. Based on your previous statements, I think this will give you a good mixture of bass, low cost, and space savings.