For some time now, I've been using leather notebook covers made by a UK-based craftsman, Fenner Benedict. Made from a variety of traditional materials, they are well-finished and of the highest quality. And of course, available easily in the UK.
However, I decided that having already got the Explorer 2 which I use with Moleskine Cahiers and Field Notes, I thought I'd try the latest Explorer 3 in Midori Passport size. Good call!
As can be seen from the photograph below, the two covers are virtually the same, apart from the size of course, and I chose the black leather for the Midori-sized one.
The Explorer range come with a cleverly laid-out set-up where a single piece of bungee cord is threaded through the cover to give four bands straight away, so that number of books or inserts can be used immediately.
But, Midori refills are a little on the expensive side for me, to use all the time so I decided to use the excellent (in my opinion) Clairefontaine 1951 pocket notebooks (9cm x 14cm) and trim down to the Midori size. Basically, I just trimmed about 7mm or the top and bottom of the notebook and it fits just fine. Because the Clairefontaine books have more pages than the Midori ones per se, I made the cut-down version a fraction shorter so that it wouldn't be difficult to fit into the cover.
I also make my own notebooks for both, especially if I want a 'disposable' one with perforated pages - shopping lists etc. Using thin card for the cover and either QC or Ryman's Bank papers for the pages. Very simple to produce.
I realise that there are loads of similar options out there, from many different countries but at least I thought you should be aware that the Clairefontaine notebooks, and of course any similar stapled ones, can easily be trimmed to fit the Midori-sized covers. I wouldn't recommend trimming ones like the Moleskine Cahiers though, because they are stitched! Goodness-knows what would happen then...!