I think it's best to use as many free channels as possible. This includes forums, communities, slack groups, chats and word of mouth.
Some of my favourite slack communities you can share your product on:
- Lenny's Newsletter community is amazing, I believe you need to sign up for his newsletter to get access!
- This week in startups by Jason Calacanis (37k members)
- Acquired (7k members)
- The Product Folks (7k members)
Sharing it on Twitter, HackerNews, HackerNoon and other sites is also great!
In case the free channels don't work out you can always try ads & sponsored posts.
Investing a little bit in ads, in the beginning, is totally fine in my opinion.
But you gotta be careful with that approach :)