Getting your hands on a digital copy of 'Tokyo' depends a bit on which version you're after, because honestly there are a few books with that title floating around. If you're talking about the Murakami Haruki short story collection, 'After Dark,' is sometimes just called Tokyo abroad, and that's widely available. Kindle, Google Play Books, Kobo—they've all got it. The ebook market is pretty centralized for major publishers. If you mean something more niche, like a travelogue or a photobook, your options narrow. I spent a frustrating afternoon trying to find a specific out-of-print digital art book called 'Tokyo' and came up empty on the main stores; I eventually had to buy a used physical copy.
For the most common 'Tokyo' novels, your first stop should be the usual retailers: Amazon's Kindle store, Apple Books, Barnes & Noble Nook. They've got the big names. But if you're outside the US, sometimes regional licensing gets weird. I remember when I was in Australia, the Kindle version of a certain Tokyo-set thriller wasn't available, and I had to use Kobo instead. Always worth checking a couple of storefronts. Also, don't sleep on direct publisher sales—sometimes they have ebook bundles or sales not listed on the big platforms. I got Mo Hayder's 'Tokyo' (which is seriously dark, be warned) directly from the publisher's site during a sale, and it was a smoother download than I expected. The file was DRM-free, which was a nice bonus for loading onto my older e-reader.