I got stuck on this too because the title is a bit tricky—there's no single book just called 'Dark Sides'. If you're thinking of 'The Dark Sides of the Sun' by Terry Pratchett, that's an early, rare one, and you'd likely need to check used book sites like AbeBooks or maybe an ebook from the usual big retailers. But if you're actually after something like the 'Dark Sides' series in romance or fantasy, that's a whole different search.
Honestly, my first thought went to 'Dark Side of the Sun' because of Pratchett, but then I remembered a bunch of indie romance series with 'Dark Sides' in the title. Your best move is to open Goodreads or Amazon and just type 'Dark Sides' into the search bar. The autocomplete usually pulls up the most popular matches. If it's a specific author you have in mind, adding their name is crucial. I wasted an hour once looking for a book just by a half-remembered title phrase, only to find it was part of a subtitle for a completely different genre.
For digital reads, Kindle Unlimited has a lot of those indie series if they turn out to be romance or paranormal. Otherwise, Google Play Books, Apple Books, and Kobo are my usual spots. If it's an older or obscure title, you might have to dig into author websites or even see if it's been uploaded to archive.org for out-of-print stuff. I'd start with a broader search and narrow it down from there.