Great question.
I've managed to find the following websites/companies located in the US:
Mosconi and Gladen products:
ORCA Design & Manufacturing (they don't seem to have an online catalogue though, so you might want to write/call them).
WoofersEtc.com - They got my exact speakers and DSP amplifier
I'm sure Ebay and Amazon sellers has these products for sale aswell.
Eton wiring harness from front seats to amplifier (I would use TechnicPNP instead)
Sound Good Stereo
Misc. Power wiring kit to amplifier, sound deadening and bare wire to RCA adapter:
You can get those anywhere I'm sure. DynaMat or FatMat for sound deadening seem popular in the US and I can attest to those being great products. I used StP for sound deadening, they are based in Europe.
Thanks for the question lsturbointeg - I've read a lot of your posts on the forum and seen your build thread - Awesome stuff!
If anyone has any questions regarding upgrading stereo in a car with base stereo, please ask away!