The Beretta 390 series had washers in the stock that would allow offsetting for a lefty. Also would make for a good light recoil for a young shooter. The only mfg. I know of that has stock cast ability.
Ajax- had the same experience myself. I started my son on a 20 gauge 1100 sporting clays with a Jack West youth stock. When he got older, he went to a 12 gauge 391 Reduced Length Target (parallel comb). In each case, all that needed to be done was to put the original 1100 wood back on and replace the RL with an adult length 391 target stock. Both youth stocks could be tuned right or left handed with shims.
Benelli came out with a LH M2 field grade model last year. I purchased one and love it. synthetic stock in black is what is available right now. Cabela's has them listed for 1449.99. Check with the store in Hamburg PA 610-929-7000. Ask for the new gun counter when calling and let them know that you are interested in the LH Benelli M2 item number 03717877. If you dont live in the area you can buy over the phone and have it shipped to your local FFL.