well it all depends on how much you can sell your current setup so we know how much you have.
the best thing for you would be to get subs designed for very small enclosures. those also happen to be the best sounding and also unfortunately the more expensive and require more power to get the same volume as your fosgates.
take a look at this site.. they have good prices somewhat varied selection:
http://caraudiozone.woofersetc.com