I've picked up copies of 'One-Punch Man' in so many places that my shelves look like a little map of my book-hoarding habits.
If you want new English-language volumes, start with big bookstores like Barnes & Noble (US), Waterstones (UK), or Kinokuniya if you're lucky enough to have one nearby — they usually stock the latest volumes and sometimes omnibus box sets. Online retailers that reliably carry print manga include Amazon, the Viz Media shop (they publish the English release), Right Stuf Anime, and Bookshop.org for indie-store support. For Japanese editions I use CDJapan or YesAsia when I'm hunting for original tankobon or special editions.
If you're after cheaper or out-of-print runs, check eBay, AbeBooks, Mercari, and local secondhand bookstores. Comic shops and conventions are great too: many dealers bring back issues and box sets, and you can physically inspect condition before buying. Pro tip — verify ISBNs and the publisher so you get the English translation (or the Japanese edition if that’s what you want). Happy hunting — there’s something so satisfying about cracking open a fresh manga volume on a rainy afternoon.