As was mentioned, Stag makes a good left-handed AR, as well as Rock River Arms. The downside, like you mentioned, is finding one!
Fishbed mentioned the shell deflector and that point is crucial for lefties. If you find an entry level rifle, some were made what we call "slick sided", which means they lack a forward assist, shell deflector, and dust cover, as cost cutting measures. If you decide to not wait for a left-handed AR (which it sounds likely based on your words) and suddenly find a righty available, pay attention to the upper configuration and make sure it has that shell deflector.
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