I know of this website that gives you access to every mailing list on the market from every vendor - all in one place. Some pay $50+ to get its location, but I give it to you here for free...
Why is this website a secret? In truth, it's not.
In fact, google "buy mailing lists" and you'll find it right there at the top of the most relevant results. But most marketers don't realize that an index of mailing lists exists and don't even bother trying to find it.
I've learned of enterprising individuals exploiting this knowledge gap and charging $50 or more to reveal this "secret tool" to you. I've looked into this scheme and there's nothing technically illegal about it, but it seems wrong to have to pay money to get a web address.
Please spread the word that the mailing lists search tool is free and available to any marketer who is looking for mailing lists.