There are a few which can be purchased, but they're very expensive. I can't really blame the artists/creators, because it seems as if a great deal of work goes into these, with people using 3D printers, cutting and painting wood, buying miniatures, adding lights, and of course, coming up with a concept, the artistry required, and the details to make it happen.
Here's one that's for sale on Etsy.
Here's a DIY Harry Potter book nook.
Here are a couple of very brief (mere seconds) videos with some finished book nooks.
Also, I highly recommend that you read this article from Bored Panda, which shows 33 of the book nooks (sometimes known as bookshelf inserts). It's one of the numerous articles I read which spurred my interest. Also, this one from the BBC talks a bit about the use of 3D printers.
And if you want to see how some of these are made, here are three videos that show one made from cardboard, one made from bricks (LEGO variety, I assume), and one constructed from wood and war gaming terrain kits (I'm not knowledgeable about such, but the creator says that they're from Games Workshop and are available at tabletop board gaming stores). As you can see from this first one, some of them are quite involved and take a lot of time and effort to make.
I'm sure there are many other ways to build these. The possibilities seem endless, and it looks like so much fun!
If you build your own book nook, please share. I'd love to see the finished results.