The new FNH SCAR rifles are fully ambi & have polymer parts(black, desert brown). They are not cheap but are available in 5.56x45mm & 7.62x51mm(.308).
See the US FNH website for product information. POF,
www.POF-USA.com was the small firearms company that sells LE/military model ARs & M4s.
As posted, for a "full" left hand AR system; a Stag Arms rifle may do best.
I use the great website;
www.GunsAmerica.com often.
ClydeFrog