I built a Stag lefty M-4rgery for my boy, he likes it a lot better than the M-4 he has to shoot for his AF Security Forces job. He has an ambi charging handle and safety on the one I built him and really misses the ability to access the charging handle with his right hand, he's hoping they'll let him add one when he gets to his next duty station. I bought the upper and bolt from AR15.com and a DPMS barrel from Brownells. Funny thing about their upper is that the dustcover flips up instead of down, has to due to the bolt release location.
DPMS makes a lefty upper, or used to, they just mill out the left side of the upper that looks like their lo-pro upper. They left the brass deflector and ejection port pin-hinge on the right side. Kinda goofy looking.