We have a few tips for tying a balloon knot by hand. However, for tying 100 balloons it might indeed make sense to get a device to help you with this. There are several balloon tying devices, but the one that received the best reviews from balloon professionals is the "Knot-a-Balloon" device. It's a small plastic gadget that you fit over the pointer and middle finger of your hand.
If you google for "Knot-A-Balloon", you'll find several stores that sell it, for example this one.
