It's surprising how many of us are always looking for new customers, when sometimes, contacting people who you already have a relationship with works best.
Even if it's your first business. Maybe you know a few folks from your previous job, an internship. These people already trust you, know you and interacted with you before... the easiest way to get your first customer, I think, is through the network you already have.
Browse through your phone and email contacts. Your first customers and users might be sitting in your pocket as we speak :)
We would not have started the company without that deposit and I would have done something else in that case.
Anyway I think you should know who your first paying customers are going to be before you start anything. But don't take anyones word for anything. Lots of people will say "yeah I'll buy that if you make it" but not everyone will follow through.
Through running a newsletter that provided similar content and promoting there.
Next time I'm much smarter, seek partnerships and even better if your product is an upsell, then you can have others sell it for you
The product(s) being of high quality still have sales after some years.