One of my go-to ways has always been to search Amazon. This is honestly so daunting, though. I've spent hours doing this. Sometimes I'll find gems, sometimes I won't. It's really hit and miss. There are a couple of sites though that will help with the search, though. OHFB is a site I used to use a lot. I think it's Kindle only. What they do is highlight different books that are currently free. I will say that if you use this, you need to watch before you buy. The books are free at the time they are posted, so when you go to buy, they may not still be free. So just double check. I've had a time or two where I just clicked blindly and ended up buying a book. Luckily I don't think any of them were more than $3. One thing I really like about this is it's not just fiction.They also post deals on cooking books and other varieties. If you've got time to really hunt it's not bad. Like I said, I found some gems doing this.
A similar site to OHFB is Bookbub. This one has something for all devices, not just Kindle. The main thing with BookBub is it's not just free books, they also post books that are discounted. What sets them apart is that they try to send you deals based on your taste preference. I haven't used this one a ton because while it does offer some free books, I think it's primarily just discounted books. I did think it was worth a mention, though, since it has some on there.
If you are a big fan of classics, then I highly recommend checking out Project Gutenberg. For those of you who don't know, this website is completely run on donations. They work to compile fiction and non-fiction books whose copyrights have expired. Once they've expired, the books can be provided for free. I believe right now they have around 54,000 books. I actually spent a glorious amount of time last night when I couldn't sleep looking through their collection. The site is a little confusing to get through, so I've linked you to the bookshelf. You can then narrow down the search from there.
Obviously, I can't write this post without talking about Libraries. I was slacking and didn't have a library card, mainly because I'm supremely lazy. However, I remedied that and went and got a library card today. I was actually really impressed with our small township library. They had a pretty decent selection and also have an e-book program. I got six books I'm pretty excited to read. Plus, now I don't feel like a giant hypocrite for promoting libraries without having a card myself. I'll do a quick Book haul later tonight or tomorrow.