Most people found them and discussed them on Slashdot, I think - certainly I first read How to Start a Startup (http://paulgraham.com/start.html) after my friend/co-founder found it on Slashdot.
A few months after How to Start a Startup was published, the first batch of Y Combinator was convened - http://paulgraham.com/sfp.html
It went so well that they ran it again that winter, then kept running it twice a year.
One of the startups in that first batch was Reddit.
There's some story about pg wanting the Reddit founders to make a subreddit for talking about startups/tech to be a community for people interested in YC, but for some reason it didn't happen the way he wanted so he decided to build his own forum, initially called Startup News (https://news.ycombinator.com/announcingnews.html), then soon after became Hacker News - https://news.ycombinator.com/hackernews.html.
The objective of Startup News/Hacker News was twofold - as real-world use-case for the Arc programming language he was creating, and to be a place where people interested in startups/tech could come to read interesting articles and discuss interesting topics, and then maybe eventually become interested enough in startups to apply to Y Combinator.
Some more detail here:
https://techcrunch.com/2013/05/18/the-evolution-of-hacker-ne...
There was a /startups or similar channel on reddit, one of the first YC companies, and they managed to attract people over here.