does anyone know where I can purchase a huge styrofoam...so I can make a molding of my trunk to make a custom subs.

Why would you try to do it with a huge piece of styrofoam? I'f your'e intent on making an exact molding of your trunk spray it down with pam and fill it with expanding foam (do multiple layers so it doesn't break anything)....most guys don't do that just so you know They just make a template in the shape of the floor, build their subs in, then make a template of the top of their sub/amp rack...much easier this way.

i'm goin to fiberglass it.

C'mon man, gimme more details than that. What part of your trunk are you fiberglassing? I don't quite understand why you would want a molding of your trunk to make a fiberglass enclosure. When you make a fiberglass enclosure you do most of the work inside the vehicle. As in you lay your glass directly in the car. OZ_GS has a great example in here on a thread called "Todays Project" or something similar to that.