A lot of guys are reloading 3/4 oz ( for 12ga, and 20ga ) at around 1200 fps ...its a typical 28ga load - and in a heavier gun its a very soft shooting shell / and as clay target shooters - we're cheap / so we're saving on our shot cost....
Even if you buy shells retail ....it really shouldn't be too hard to find 12ga shells at 1150 fps probably in 1 oz ....or 7/8 oz loads ( a typical 20ga load ) at 1200 fps for a 12ga. Companies like Remington, Chedite, Rio, etc all make light 12ga "target shells". But you have to read the box carefully ....I see some Remington Gun Club "light target loads" around but they have 1 1/8 oz of shot...( and that is not a "light" load).
In 12 ga shells..( for light loads)
Look first for 7/8 oz or 1 oz payloads
Then look at velocity ...and keep it at or under 1200 fps..