If you’re short on money for ad spends, but have time, you can get involved in those communities as well. DO NOT go in spamming your app, you’ll get kicked out.
Answer questions. Help people with their concerns, problems, and issues (remember, it’s about THEM and THEIR needs, not YOU and YOURS).
When someone has a problem your app solves, you can give them that as a solution (become trusted first; use the old Bob Burg gem: “know you, like you, trust you”…and then you can get their business).
If you don't have a trial on your app, I suggest you do one. If you can wait 30 days, that’s probably the best (a longer trial version or return window gives users more time in their minds to return or cancel, and they're less likely to do it).