If you're after a physical copy, I'd hit up Bookshop.org first—they've been my consistent spot for Kellerman's hardcovers on release day. Their shipping is decent and they support indie bookstores, which feels good. Amazon is always an option if you need it tomorrow and don't mind the big A, but I've had better luck with Barnes & Noble's website for signed editions in the past. His publisher's site, Penguin Random House, sometimes has pre-order bonuses like chapter samplers.
For digital, Kindle and Kobo are straightforward. The audiobook is trickier; I listen on Audible but know it's on Libro.fm too. A library app like Libby is worth checking, though you'll likely be on a waitlist for the new one. I grabbed 'The Ghost Orchid' the day it dropped from a combination of Bookshop and my Kindle, honestly. It was simpler than chasing a signed copy from a local store's event.