Product Hunt is a good place or maybe some beta sites like BetaList to get started.
The longer and "cheaper" way is to write your own content, which gets attention. It depends on how connected you are already. Some people have already a huge interested audience.
But you need to either build an audience, pay for adverts and be optimistic, or be constantly promoting it in all appropriate venues. Having google-alerts, and twitter-searches, to make sure you can join suitable threads promptly is very useful too.