Buck shot is just really really big lead shot (just like you use for birds...except these things are huge, and you've only got 8 or 15 pellets in one shotshell...instead of several hundred lil' ones). Slugs: you're using just a regular shotgun barrel right? In that case, I'd look at some brenneke slugs or a new thing called (I think federal makes 'em) called "Tru-ball"...if you can't find those look for anything labelled "foster style". AAANNDD...that part no one want to hear is that you're gonna have to try several different types to see what you're particular gun shoots best. Also, a good recoil pad is probably worth the investment if you don't already have one.

Remember this also, Sabot-type slugs are made for rifled shotgun barrels, they'll work but you'll likely get better results with the Foster-type slugs.