In my experience, simply finding early stage startups is hard. Oftentimes they don't even have a web page during the earliest stages, and if they do, you likely wouldn't be able to find it, and then it might not have any contact information, and if it does have contact information, you have no idea what they're doing, to know if it's worth contacting them or not.

I think it might be easier to find early stage startups if you try to aggressively network in that space, but not being much of a networker I couldn't say. I guess I should add that I'm in Seattle which seems to have a much smaller startup scene than the Bay Area.

I got my most recent job through AngelList but in that case I didn't use it to research companies, I just spammed every startup listed in Seattle that had an opening I was remotely qualified for, and then just talked to whoever responded. Before that I had basically scraped CrunchBase for startups in the Seattle area that had received funding at some point or another. I applied at a bunch of companies, but I don't think I got a single interview out of it.

This is a long winded way of saying that, yet, in my experience getting into a good startup is pretty much just luck.