The good thing about a good bookstore or library isn't finding the book you came in for, it's discovering the book you had to have that you didn't even know existed/you were looking for.
Such discoveries are made by hunting for them. You can go online, through publishers catalogs, or read reviews of books. But that doesn't cover everything published. And is still a hit and miss approach. And holding a book in your hands permits you to thumb through it.
I don't find much at the big barn bookstores (bookstop, barnes & noble, or borders) although 20 years ago Bookstop had books that no other store did (in the era of Walden and Barnes & Noble chains and independent stores).
Now most the chains all stock the same 50 titles in any section.
I agree, there is something about browsing a good book store and stumbling on a great book. Many of my favorite novels were "accidents" like that.
That can still happen at times in the big barn stores. For instance, my local Borders has quite a selection of military history and theological books. More than you would ever expect.
While I do enjoy browsing used book stores, I rarely have time, and am increasingly finding that I CAN discover the books I didn't know existed and simply have to have. When you're reading something online about a topic that interests you, you often run across mentions of books related to the topic, and then you can Google the title of the book and also the author's name, and often find a great deal written about it (sometimes even better than "thumbing through"), and that information often includes references to yet more books or authors that you've never heard of. Then you can hit used.addall.com or www.bookfinder.com and get the book(s) you've discovered within a few days, no matter how long they've been out of print or how obscure/specialized they are. My personal library includes a number of books that I discovered this way, which have been out of print since the mid-1800s, and which I would likely never have discovered by browsing around used bookstores or borrowing libraries.