There are lots of ambidextrous parts that you can add to a .308 AR that will help you out, but at this point, nobody makes a true left-handed .308 AR. DPMS might have something ambi for the -15 platform, but Stag/CMT is the only company that make a true upper receiver that ejects from the left side (special bolt carrier, upside-down ejection port door that's on the left, special upper to run the cam pin).
There are a couple .308 AR rifles that go above and beyond regular ambi parts, too, with a couple custom touches for left-handed shooters. The SIG 308 incorporates a bolt catch on the right side. Knights Armament does a few special things on their SR-25 platforms, too. You're into some serious big bucks of you get into a KAC rifle, man.
Ive been looking at their new SR-25 ECC (Enhanced Combat Carbine)...
http://www.knightarmco.com/portfolio/sr-25-ecc/