I recommend the Mossberg slug barrel/scope combo. You can order them from Walmart for like 185 or so. I have been using this setup on my 870 for the past two years.
There are almost NO options for non-toxic slugs. The ones I've heard about behave terribly. LtDann went through this during deer season and you should read his posts (I think under the shotgun thread). The best option is to use a rifled barrel and copper sabots but these will NOT shoot in a smoothbore. I repeat: do NOT use lead rifled slugs at FHL, it will get you cited.