Path finding libraries are not too common for general use. This is because the algorithm interacts directly with your world. The algorithm can't know in advance how you'll lay out your data structures for holding your world data. The amount of user required implementation for these algorithms would likely override the usefulness of providing the algorithm in library form. The more complex the algorithm, the more work it would be to make a generalized version for a library form. So you're less likely to really get what you want, the harder-to-implement algorithms won't be in library form. There are some libraries available, though I haven't used them, so I can't say how much work they are to use. Personally I would rather implement them myself so I can learn how they work and ensure they're properly integrated.
It may be useful to look at the libraries to get some implementation tips, but I don't think these will be implemented in the same way you would implement them. Since they need to overcome the issues I mentioned earlier, they might have some strange implementations.
For 2D, Slick2D has some built-in path finding for tile based games. StackFrame implements some simple pathfinding algorithms with PathFinder.