The third edition is out of print and is hard to find. I don't know of any stores that have copies. There are some sellers on Amazon asking absurd prices, around $98-$200. They are looking for suckers, so don't spend that kind of money.
There are first and second editions available on Amazon for reasonable prices. Each edition has some updates and additions, but I think you will still find the earlier editions useful for exploring PA streams . Pine Creek in the canyon is still Pine Creek in the canyon. In other words, some things have changed regarding our streams, but many things have not.
You can also get the book from your local library. If they do not have the book, you can get it through inter-library loan.
I hope to come out with a 4th edition at some point. But when (and if) that will happen, I do not know, and I will not speculate on that.
If anyone has suggestions about ways to improve the book, corrections, additions, changes in design, format, improving user-friendliness etc. for the book in a future edition, please send me a PM.