I was in this position recently. I found some at grabagun and got a few Aguila 7.5 shot ($7.69 per 20rd box) and slugs ($11.99 per 20rd box)...not the best prices I've seen but not the worst either.
Have a pair of Mossy 500's. Both are stoked with #4 and Deer slugs. Have a box of LE 00 Buck on reserve, a few more of cheap #7-8 WallyMart packs. Not feeling the pain of any underarmed ammo selection. Never tried mini shells, some guys like them. The proof is to test those in your SG. Stoke that defense arm with available rounds if in a bind. Short range is optimal for the Defense Shotgun. Glad you found a bit and info here. Good luck.