If you're hunting for a paperback of 'The Wild Robot Protects' (book 3), you've got lots of solid options depending on whether you want brand-new, discount, indie-supported, or used copies. Start with the big online retailers like Amazon and Barnes & Noble — they usually have new trade paperback editions in stock and offer fast shipping or in-store pickup. If you prefer supporting independent bookstores, Bookshop.org and IndieBound are great: you can order online and have the sale benefit a local shop, or use IndieBound to find and call nearby stores that might have it shelved.
For cheaper alternatives, check AbeBooks, ThriftBooks, and Alibris for used or older paperback editions; eBay can also turn up good-condition copies and seller bundles. If you're outside the U.S., check Waterstones or Blackwell's in the UK, Indigo in Canada, or your regional chains. Big-box retailers like Target, Walmart, and Books-A-Million sometimes carry popular children’s series in paperback too — both in-store and online.
If you're in a hurry, call your local bookstore to hold a copy or use curbside pickup where available. Libraries or digital lending services like OverDrive/Libby can tide you over if you want to read it right away, although that won't get you a physical paperback to keep. Personally, I love snagging a paperback from an indie shop when possible — it's satisfying to flip through the printed illustrations and know a local clerk helped make the sale.