The first few will definitely be through your network, friends and family referrals, so it's important to build your connections. Apart from that you can have a good landing page, and run ads redirecting leads there.
For one low-ticket subscription product aimed at Moms, we contacted 100s of Mom bloggers, gave free samples and asked the to write about it.
Work very well. This was a few years ago.
Zapmail : Slack as an email service