For target work,3" shells are counter productive AND overly expensive. Use 2 3/4" loads, 1 oz or 1 1/8 oz will do fine for the clay sports in a 12 gauge, 7/8 oz in a 20.
Try Ammo Man, Midway, Cheaperthandirt, Gamaliel SHooting Supply, etc.
Also, most clay shooters end up reloading. Not only is it cheaper per shot, but one can tailor a load just for one mission and shotgun.