Recruit them manually. This thread has a lot of great tips.
https://news.ycombinator.com/item?id=6041765
Being niche is okay, but being sustainable is vital. There are many ways to articulate this, and I'm not an unqualified fan of pg, but he did capture this wisdom very well in "Do Things That Don't Scale" [1]
[1] https://news.ycombinator.com/item?id=6041765
Recruit them manually. This thread has a lot of great tips.
https://news.ycombinator.com/item?id=6041765