I tried googling it up and got nowhere.
I couldn't find any published research which showed any difference between the two chiral forms, nor could I find and difference in how they are manufactured. I suspect that if there is something distinct in the actual vitamins themselves, the right stereoisomer is just as effective as the left stereoisomer. If not and the N-Acetyl-cysteine is a mixture of the two and only the left is bio-chemically active, then the N-Acetyl-cysteine pills would still contain half N-Acetyl-l-cysteine. I would buy whichever is cheaper.