Retrosub,
What kind of shotgun is it? Can you attach a rifled slug barrel? I have an NEF pardner and I got a 20 gauge, rifled barrel for it for about $80 from the factory, including shipping.
I shot the copper solids through it and they seemed accurate enough for deer. I only shot a few though, given the high cost.
I too live in the lead free zone.
Another major ammo company makes sabot coppers but the name escapes me. Federal, maybe? But i have heard of no copper slug for smoothbores.