I would like to practice tracing a variety of images in Adobe Illustrator moving from basic to advanced in complexity. Is there any single place that stores like a pack or list of images like these? My only other option seems to be browsing the net and picking out images as they appeal they to me.
As DA01 has mentioned there isn't a place like this or at least not one that either of us are aware of.
As you've mentioned your other option is to browse the net for images, and that's really the way to go. If your strategy is to go from simple traces to more complex ones then my recommendation is to think of something that matches that description.
For instance, something simple would be something very geometric. Maybe a building, or balloons. Building in complexity you could move to vehicles or other things that have more complexity but are still rather geometric.
I would just continue this way, of thinking of things that are harder and harder to do. I'd imagine something like a tree or something with fur would be rather difficult to do in the end.
Either way thinking of something to trace and then finding an image that you'd like to practice on should be pretty easy and straight-forward.