what kind of slingshot? I used to shoot a lot at a kid; we'd stick quarters between the boards in a backyard table and shoot at them. I got to the point where at about 30 feet I could hit them maybe 75% of the time - I stopped shooting when a ricochet went over the fence and shattered a neighbors glass back door. I've been interested in getting back into it, because it's cheap and fun. I never used marbles much, mostly ballbearings or candy (because the candies are smooth, fairly even in weight, and will shatter instead of breaking things). I once hit a robin in the head... with a candy called a robin's egg.
I used to use a wrist rocket that I got at wal-mart, then a few homemade ones. I currently have one, forget the origin, but it's got black bands and trying to draw it back makes me feel like a wimp... no wrist brace + a really strong band makes for a really hard shooter, but kinda hard to aim. Did you get into it because of Joerg Sprave, or just because?
I buy all my stuff at motorcyclegear.com, formerly newenough.