If you're hunting for a gamemaster book online, I can walk you through the places I always check first and why.
My go-to almost always starts with the big storefronts: Amazon for quick shipping and predictable returns, and the publisher shops — like the Wizard of the Coast store if I need a D&D core book or Paizo for 'Pathfinder Core Rulebook' items. Those official stores often have special editions, bundles, and preorder windows that show up nowhere else. For PDFs and print-on-demand, DriveThruRPG and DMsGuild are lifesavers; I buy a lot of supplements there because I can read them instantly on my tablet and print pages I need. Humble Bundle occasionally drops RPG bundles, too, which is great when you're trying to expand your digital library without breaking the bank.
If I want something out of print or cheaper, I head to used markets: eBay, Noble Knight Games, and local game store websites. Noble Knight is amazing for older boxed sets and condition notes, while eBay can score rare finds if you’re patient with auctions. I always cross-check ISBNs and edition numbers — sellers sometimes list the wrong edition — and I read seller reviews closely to avoid surprises. For international purchases, I’ve used Bookshop.org to support indie bookstores, and regional shops like Waterstones, Forbidden Planet, or Philibert (in France) depending on where I live. Lastly, don’t forget to check PDF vs. physical prices, watch for sales, and consider buying from your local game store online when possible; supporting them keeps the hobby spaces alive, and a lot of LGSes will hold items for pick-up at conventions or give member discounts.