When I went to New York Comic Con this past fall, I met representatives from some of the major publishing companies. That's where I learned the trick to these free early copies. Make sure you mark in your profile that you are a teacher and/or librarian (if you are) because the publishers are more likely to approve your request if you have a good reach. Below are the three sites I have joined to request ARC.
I have been a member of NetGalley for a few years now. It was the first site I learned about back when I was blogging about how to save money. Many of the large publishers will use NetGalley but be aware that I've been declined as many requests as I've been approved for during the time. Some newer authors will be listed as "read now" and don't require a pre-approval.
I learned about Booksprout directly from an author's Twitter account. You can search through available ARCs but you can allow follow an author you like. I now get notified when that author I enjoy has new books available.
I learned about Reedsy from someone at NYCC. The site has an interesting variety of books but it appears that only one reviewer per book, so it's first come/first serve. You can also see the books that have already been reviewed. The one thing I found strange with Reedsy is that people can leave you tips for your reviews. If you receive a tip, you must connect a Stripe account to withdraw the funds.
If you're not interested in pre-release books or agreeing to review the books, you can also look into options like your public library and Amazon for free eBooks.
Depending on where you live, it may be different. Here in Philadelphia, we have Overdrive and Hoopla. The library has a specific number of digital licenses for each book and if all those books are "out" you can add yourself to the waiting list. You can also search for books the library doesn't own or pre-release titles and recommend the library purchases them. When they do, you'll automatically be able to borrow the title or be added to the wait list if it is a popular pre-release.
Amazon has a subscription program where you can get unlimited books. The subscription costs $9.99 per month but if you read a lot, it may be worth it. They also offer a free trial month. I'm a little torn on Kindle Unlimited because they have a huge selection of books but often new releases and popular books are not included.
If you are an Amazon Prime member, you can filter ebooks to find titles that are eligible for Prime Reading. These titles can be downloaded for free. The selection is more limited than Kindle Unlimited, but you can often find some decent titles.