I usually keep a 100 rd bulk box of target ammo on hand for clays or cheap blasting along with one full load of buckshot. i recently went to the local gun shop and bought all the aguila mini buck and mini slug they had since my KSG feeds them, nobody else seems to want them, and they have been discontinued. So currently I have 280 12ga mini shells and 100 target shells.
edit- as for cheap slugs try gunbot, i have found (have yet to buy) slugs for $.51 a shell.
Sgt. of Marines, 5th Award Expert Rifle, 237/250
1st Award Expert Pistol, 382/400
D Co, 4th CEB, Engineers UP!! OEF 21JUN-20SEP2011