I think some other people in the thread already said something similar, but wouldn't it be best to grow a niche audience organically?
At the end of the day, you (probably) want a niche audience because they're easier to monetize but if you buy/fast-track your way to that audience and its inauthentic or not genuine, wouldn't that cause trouble with monetizing it later on.
I could be wrong, I'm somewhat new to IM. Any money I've made online has been "accidental" (people reaching out vs. me deliberately looking to convert) but in my opinion you should try to provide actual value to your niche and then they'll be good customers later on, even if they are a smaller amount in numbers.