I'm glad someone's asking about this, because it's such an important topic. The book you're likely looking for is 'My Body Belongs to Me' or something similar, like 'I Said No!'. A direct Google search for 'good touch bad touch book' will pull up a few different titles, but the classic is often the one by Jill Starishevsky. I found my copy on Amazon a few years back for a friend's kid. It's pretty straightforward to find there, and you can usually peek inside the pages to see if the illustrations and language are right for the age you need. Barnes & Noble's website also carries a selection, and sometimes you can find used copies in decent condition on AbeBooks or ThriftBooks if you're okay with that.
Honestly, the hardest part isn't finding where to buy it, but picking the right version. Some books are very direct and narrative, while others are more workbook-style with activities. Reading the reviews on those retail sites is key—parents and educators often share exactly how they used the book and what age group it resonated with. I'd avoid random websites that aren't major retailers or publishers, just to be safe with payment and getting the correct edition. The price is usually reasonable, and it's one of those books that feels worth every penny once you see a child understand the concept in a gentle way.
One thing I noticed is that local independent bookstores often don't keep it on the shelf, but they can almost always order it for you if you call. Sometimes that feels better than giving more money to a giant corporation, you know? Plus, they might have a recommendation for a similar title they prefer. In the end, having the physical book seems better for this topic than an ebook, so you can sit and go through it together slowly.