IIRC, slugs are not legal for hunting in New Jersey, so they would be difficult to obtain. Amazon, maybe?
I buy a lot of my ammo from Aim Surplus.
For smooth bore shotguns Remington Sluggers are pretty good.
https://www.aimsurplus.com/product.aspx?item=ZAAS12MRS&name=REM+AMMO+SLUGGER+12GA.+3%22+1760+FPS.+1OZ.+RIFLED+SLUG+5PK&groupid=4966