How do you prospect and find leads?

This doesn't directly answer your question about finding contact details for cold emails, but hopefully still useful tips about cold reach outs.

1) Find companies to target.

You mentioned you know your ideal customer, but you still need to find companies that contain those profiles. Given you're a Slack app, would recommend G2 and stackshare as a starting point:

https://stack.g2.com/slack
https://stackshare.io/slack/slack

2) Finding who to speak to in those companies.

Use LinkedIn to search up the leads that match the roles you outlined (tech / team leads, PMs, or management) and either reach out to them directly on LinkedIn or try and find a corresponding twitter profile or email if it's listed on their LinkedIn account. I would say reaching out direct on LinkedIn is fine though to cut out some extra admin.

3) What to say in your cold reach out.

This is the part that takes a bit of time to figure out with trial and error. At the start everyone gets a low response rate, don't be too disheartened. Think about the main sales hooks of your product and connect it with customer pain.

If you're open to this, the first interaction doesn't have to try and be a sale. Maybe frame it as you're doing some research to learn more about how PMs currently hold their stand-ups. This can be useful for you to a) continue learning about your customer to help build your product and b) mention at the end of the call what you're building and ask if they'd be interested in learning more. People are often more willing to talk about themselves a bit rather than jump straight into a sales conversation.

This isn't as automated as scraping emails and sending out cold emails to a large mailing list, but when you're trying to get the ball rolling it can be better to do things that don't scale (inspired by: http://paulgraham.com/ds.html)