I use a 200A GM compatible 3 wire marine alternator on my boat to power a 4000w RMS/6 amplifier sound system.. and it never falls below 13.5v even when the bass is knocking your teeth out. I picked it up from a place called PowerMax USA on ebay for $170. Awesome unit.
For other electrical units such as starters and off the shelf alternators, I've been using a place called DB Electrical for years.
Yeah.. it's aftermarket Chinese built stuff, but it works and lasts pretty damn well and at a fraction of the price of OEM or others with a solid warranty. I recommend them highly. Their catalog is amazingly extensive.