People who are good at something you want to be good at. People who invented or popularised techniques you like. Many of them I've come across have published their emails, if nothing else as a "if you have feedback on this book, reach out to name@example.com".
You can build your own list (capture emails on a subscription
form on your website like I do), some people buy them from people who sell such things (although Im certainly not advocating for this, just answering your question), or use tools like LinkedIn sales navigator, etc.
I'm not advocating, or not advocating for any of these approaches. I'm just answering the commenter question.
Also, due to the downvote I suspect at least one person is focusing on the fact that yes, in some circumstances buying contact lists can be annoying and frowned upon.
In other cases, it's how business is done. I'll give a real world example from a guy I actually met. Say you're the head of sales for the largest independent lolly pop manufacturer in the US. How do you let all the corner stores in the US that they should sell your lollies? Buy a list, send them letters/email, whatever. People can be annoyed by it, but it's what keeps the post office in business and is half of what all marketing and sales folks do half the time.
Another example is in the insurance industry, it's common practice to sell your client list when you retire in many of those spaces.
Again, I'm not Advocating or not advocating for any of this, just answering someone's question.