Most folks just check the big ebook storefronts, but the real trick is knowing which ones tailor to your reading habits. I lean heavily on BookBub's daily email—it's not just a list, it cuts across different retailers and actually learns what you like after a while. The freebies are usually from smaller or indie authors, but I've found some solid genre fiction that way, stuff I wouldn't have paid for but ended up really enjoying. Library apps are the unsung hero here, though. Services like Hoopla and Libby don't have 'daily deals' in the same flashy way, but their entire model is free access with a library card, and the 'available now' section refreshes constantly. It feels more sustainable than hunting for temporary price drops.
Sometimes the best 'deals' aren't on books that are permanently free, but on the first book in a long series that's been discounted to zero for a limited time. I snagged the first 'Murderbot' diary that way ages ago, and you better believe I bought the rest. For that, I lurk on r/FreeEBOOKS on Reddit. It's messy and full of self-pub, but users are pretty good at upvoting the genuinely good temporary offers from bigger names. The discussion in the comments can tell you if it's worth the download, which the sterile store pages never do. My Kindle library is full of stuff I got from there that I still haven't read, but it's a nice feeling having them.