The California Air Resources Board (CARB) has issued it's list of aftermarket exhaust systems legal for sale and installation in California. For the Harley aftermarket the list is short, below are the exempted systems. The full document can be found here.
BUB Enterprises Inc. Eight exhaust systems, both slip-on and complete, covering 2002-2010 FL motorcycles.
Cycle Shack. Three exhaust systems covering 2008-2010 FL and Dyna.
Thunder Heart Performance. Four heated oxygen sensors.