I've been going through this a bit with a tool I've thrown together for doing feedback on video game footage (https://www.volt.school/)

I've had success with cold outreach to users on Reddit (random DMs to them) and Discord communities. Now neither of these are going to help you (unless you've also got a video game product!) but I think it demonstrates that you need to go where the users are and not be afraid to tell people about it. If the product is genuinely useful to the target audience then you're helping them out by letting them know about it, the is especially true if it's free.

Finally, I try to think of marketing like I do with programming. You're solving problems and the solutions to those problems don't always come to you straight away. Sometimes you think of the solution when you're in the shower or on a walk. Sometimes there's more than one solution, or one that works much better than another but you only discover it later. The important part is to keep thinking about the problem and to keep turning it over in your head.

Really entirely depends on the product / service / industry. Without specifics, we can only give you broad answers. You could always pay for a list from a site like https://www.dataaxleusa.com/