I get excited whenever someone asks where to read 'Wonder' online—it's one of those books I shove into people's hands like a favorite snack. If you want a legit digital copy, start with your local library app: Libby (OverDrive) and Hoopla are the big names and many libraries have both the ebook and audiobook of 'Wonder'. You can borrow it for free with a library card, and if it's checked out you can place a hold.
If you prefer to own it, Kindle, Apple Books, Google Play Books, Kobo, and Barnes & Noble sell ebooks and often let you preview the first chapters. Audible and Libro.fm carry the audiobook if you want to listen on commutes. There’s also Scribd, which sometimes includes 'Wonder' in its catalog if you use a subscription.
I also check publisher excerpts—Random House often posts sample chapters—and secondhand sellers like ThriftBooks for cheaper physical copies. Whatever route you pick, supporting legal sources is good for authors. If you’re hunting a quick sample, Google Books usually has a preview that scratches the itch.