Get the lightest 12 gauge loads you can find. I have seen 7/8 ounce loads available. Federal makes some excellent “Flight Control” buck shot loads that are about the best out there, and they are lower recoil than the standard buckshot.
If you have a friend who reloads, have them reload you some ultralight loads. You can get reliable functioning in manually operated shotguns with 1 ounce of shot at about 800 feet per second. I recall that this requires about 10 grains of Red Dot powder, but as this will not operate autoloaders, you have to keep the loads separate.
Ah... Here is the perfect training load if it is stocked in your area. It is very light recoil, very quiet, and is a factory load that is not horribly expensive...
Federal subsonic 1 1/8 ounce of shot at 900 fps.
http://www.federalpremium.com/products/details/shotshell.aspx?id=728
About the same felt recoil as .410 3/4 ounce load at 1100 fps (because of a little heavier shotgun in the 12 ga.).
Thanks, that’s great info!