In case you haven't heard of Targetz.com I suggest you pop over there now, it's the most comprehensive collection of high quality targets I've ever seen, it's also completely free to download and print their entire collection.
Here's my favourite one for slingshots, I believe it's perfect for practising with because the bull's eye is 2" in diameter, which is how accurate you need to be in order to consistently kill small game and pests. The outer rings are in one inch increments, with a cross going straight down the middle, making it easy to measure your accuracy.
Here's one I think Torsten his buddies will enjoy.
I'd love to y'all competing with it in your next youtube video!