Ask HN: Niche app customer acquisition?

I recently launched a new service for monitoring SSL certificates [1], and I'm trying to figure out how to find customers.

I built this service because of a need I had, one that I thought others would have as well.

My service does couple of simple, useful things: lets you know if your certs are going to expire and lets you know if your certs are misconfigured or not working.

I initially saw the value in this type of service as being a simple way for people to set up reminders about when their certificates are going to expire. (No more need to worry about who the CA is emailing)

But now I think there may be more value in the validation piece.

Basically, I'm having a hard time figuring out how to target customers that would find my service useful.

I have a couple of targeted AdWords ads running right now and I have a reddit ad scheduled for later this week.

I've had some thought of writing an app that looks for expired SSL certs on the web and emails the domain owners, but this seems too spammy (maybe even illegal). I also thought about approaching a smaller registrar for a partnership, but I don't think any will go for working with such a new startup.

Does anyone have any tips for acquiring customers for a niche product?

BCC comes to mind, and maybe my type of service is similar in that I'll just have to wait until people with a need start looking for a solution, but I'd love to take a more active approach to customer acquisition, if possible.

[1] https://www.certician.com/