Ask HN: How to find users of specific online services or products?

For example I want to talk to people that use Jira and Trello (both) because that intersection is the target audience for the product I'm building and I'd like to conduct customer development with their help. How do I find them? How do I find their contact details? How do I reach out to them without coming across as spammy? Are there any more or less well-known online services that facilitate that?