How can i market a SaSS product build as a plugin or app that runs on slack?

There might be some documentation or podcast episodes about how Polly or Dailybot builders went about it. The thing with Slack plugins is that they kind of grow themselves, you just need to get the ball rolling first. Imagine you join a random Slack community, see a cool plugin and decide to integrate it into your own server as well. I'd probably recommend starting out with stuff that doesn't scale so well but will get you that first 1, 10 or 100 customers. Reddit is great for this as you can just browse relevant subreddits and jump into conversations where people talk about the problem you're trying to solve with your own plugin. Surfkey.io by @johancutych is great to find relevant subreddits, threads and comments on Reddit.