so this comes up a lot. For a legal route, your absolute top source is the official Tor Books website or their newsletter—they often have direct purchase links for ebooks and sometimes run sales. Amazon's Kindle store is, of course, the biggest marketplace; you can buy individual books or subscribe to Kindle Unlimited, which includes the whole series last I checked. That's probably the easiest one-stop shop.
Audiobook fans should head straight to Audible. Michael Kramer and Kate Reading's narration is legendary in the community, and having it all in one place with Whispersync is a game-changer if you switch between reading and listening. Don't overlook your local library's digital services, either—apps like Libby or OverDrive often have long waitlists for popular series, but it's free and 100% legal. I snagged 'The Eye of the World' that way when I wanted to revisit it without buying another copy.
For a more niche option, some specialty ebook retailers like Kobo or Google Play Books have them, and they sometimes offer DRM-free versions, which is a huge plus for archiving. I'd avoid any site offering 'free PDF downloads' of a series this big; it's almost always a scam or a pirate site, and Robert Jordan's estate deserves the support for such a monumental work. Start with Kindle Unlimited or Audible if you want convenience, or support the publisher directly.