06LT

Hey guy's awwesome forum, I am in the middle of doing the big three upgrade to my car. 2006 Monte Carlo LT, 3.5L engine. Now i have located all the wiring for my car that needs to be upgraded. I'm leaving the existing OEM wiring in place and am just adding onto it. Question i have will 4 gauge be enough, and my alternator does not have a negative cable on it. In a different forum people have mentioned that the alternator case itself is the ground which is bolted to engine mounts. Is there another way to connect my alternator negative to the chassis? Also is there a place where they sell aftermarket alternators for my car? I would like to buy one instead of getting one built for me?