It might be worth a drive to Bass Pro on a Saturday. They have a couple of different private companies that teach gun courses there. They also have different classes based on experience levels. The name of one company is Inland firearms training. I cant remember the others. They use the indoor...
There seems to be crazy deal right now on a case (250) of Winchester Super-x Xpert Hi-Velocity Steel shells. If you select #7's you get them for $36.99 a case.
If you can find it cheaper post...
No one makes it. Federal make 00buck in steel.
Lead free in tungsten for yotes will be in T and smaller(off the shelf).
Kent bismuth will be an option for your 20 gauge.
If using apex ammo you can step down the shot size needed considerably.