How would you find Slack users to market your product?

Well, Slack users hang out in Slack groups. I use Product Hunt and found some pretty interesting directories that listed Slack communities by niche/market.

Regarding effectiveness:

In my ZeroToUsers research , I'm yet to find examples of Slack being a channel that consistently works for founders. Saw one example of someone saying it worked when they DMed people, they got some results. But simply posting in a Slack channel, that doesn't seem to work quite well because:

a) Most Slack communities have special channels for posting links that basicallly nobody checks

b) Even if a) is not true, the nature of Slack is that the most recent post is the most visible one. There aren't algos to promote posts that received the most engagement (like with Reddit posts or Facebook groups) and that gives your post way less visibility vs. using these 2.

Google: [industry] + Slack community / communities.

I did that for 'design', came up with this - https://www.designerslack.community/