I get asked this all the time by friends and family, so here’s the way I usually help people out.
The first place I check is my local library app—Libby (OverDrive) or Hoopla—because I’ve borrowed the whole 'Percy Jackson and the Olympians' series there before. If you have a library card, you can often borrow eBooks and audiobooks for free, place holds if they’re checked out, and sometimes get longer loan periods for kids’ editions. It saved me a few bucks and I love the convenience of reading on my phone or tablet.
If the library route isn’t an option, I look at legit stores: Kindle (Amazon), Google Play Books, Apple Books, and Barnes & Noble’s Nook carry eBook and audiobook versions. Services like Scribd or Audible sometimes have them too—Audible offers a trial if you’re new, and Scribd occasionally has a rotating catalog. For physical copies I check local bookstores or used bookshops; boxed sets turn up at good prices. Also, Rick Riordan’s official site lists retailers and special editions, which is handy. Avoid sketchy PDF sites—copyrighted books there are illegal and risky to download.