Ask HN: How can I discover mid-stage statups?

This is probably a dumb question, so bear with me. In the past few years I've noticed some folks at my company migrate to some pretty interesting mid-stage startups, all of which were completely unknown to me.

I feel like I get reach-outs a lot from early-stage startups, so discoverability isn't so hard there, but I'm a little too risk-averse to take them on -- I'd prefer something with slightly more stability. It seems like I need to do some research to find those.

Is there a good site or database for this?