It seems that this simply designed notebook has become something of a sensation, if you can make such a determination according to YouTube videos and assorted blog posts.
While Midori Traveler's Notebook seems to have what may appear to be a fanatically-devoted following for a stationary product, they are by no means alone in having such, as you can see from people who use products like Moleskine Notebooks, Field Notes and Filofax, among others. There are people who devote time and energy to posting videos and blog posts telling the world how much they love these products and how much other people should use them too.
But, while the other brands I've mentioned seem to be widely available, a frequent complaint among MTN devotees is how hard it is to get one if you live in the United States. According to one video I saw, there are only four or five vendors in the USA where you can get one.
The Truth is that, while products like Moleskine are quite easy to acquire, MTNs are not impossible to find from American vendors. Here is the list I have put together, so far:
Amazon.com offers them for a reasonable price and you can see Midori's Amazon page here.
I live in Honolulu, Hawaii and my first thought was to try to find them at Shirokiya in the Ala Moana Shopping Center. While Shirokiya does stock many products from Japan, they do not yet sell the Midori Traveler's Notebook. The only store in Hawaii that sells them is Hakubundo Books, at the Ward Center. Visit their website here.
You can purchase them at one of several Maido Stationery & Gift stores. They have locations in San Francisco and San Jose. Their sister company Kinokuniya Stationery has locations in Los Angeles and New York City. Visit their website here.
Baum-kuchen seems to do its business solely via the Internet and does not appear to have a brick-and-mortar store. Their website can be found here.
Resor Shop is located in Miami, Florida and seems to have the MTN as its primary product for sale. The site looks good and you can see it here.
OMOI is located in Philadelphia, Pennsylvania and their website is here.
The Goulet Pen Company is based in Ashland, Virginia and their website is here.
This list is as complete as I can make it, with the limited information I have and there may well be other store in the USA that sell the Midori Traveler's Notebook that I am unaware of at the moment. If you know of others, please post their information in comments.
If you simply want a traveler's notebook of the same style as the MTN, but either don't want to spend that much or a brand name is unimportant to you, there are a number of people who make and sell them on Etsy.com and their products could be of the same quality as those made by Midori. The closest in appearance and function to the MTN is made by someone in Canada, known as Zenok Leather and you can browse their collection of products on their shop page.
That's it for now. I'll add more when I can.