There used to be a lot more " left handed " guns around / and a lot of the bigger companies, especially Remington and Browning, made a number of left handed special run models that were not in the catalogs / and a lot of special run guns for right handers too.
They were typically made for bigger dealers - where the dealers had to order at least 50 of them.
These days you will see these special order guns / especially in O/U's and semi-autos / where a dealer placed a special order ( like ordering fifty guns made up in a 28ga or .410 ) off of a 20ga receiver - and they won't appear in the catalogs but you'll see them for sale in the "shotgun" magazines or maybe in a used gun shop. The one I see around my area the most is a Browning Citori XS Skeet model, 30" barrels, made on the 20ga receiver either in 28ga or the .410 . The gun is only available in the catalog as a 12 or 20ga - but a big dealer in my area about 8 years ago, ordered 50 of each - and put the word out that he had them .... I bought a pair of them in 28ga and .410 / and I walked into a good used gun / pawn shop in my area about 2 weeks ago - and there were a pair of them on the wall ( almost unfired ) for $ 2,500 each ( and they both sold right away ). So to some extent - you have to keep your eyes open - talk to some of the bigger dealers in your area / stop in at a Cabellas once in a while / see what a site like GunsAmerica may have and just make the managers or owners know what you're looking for .... be a little patient .... and something may pop up.
A buddy of mine shoots a nice Rem 1100 12ga leftie - that was custom stocked by Jack West in Oregon / with a parallel comb and adj comb - not a standard gun at all ......... and he's had it for 25 years or so. His primary Skeet gun is a Krieghoff K-80 / but he uses that Remington as a backup ( or rain gun in nasty weather ) and as a " travel gun " because its relatively inexpensive compared to the K-80 .....