I've been working on this for about a year and a half now, though I was able to get my first customer just a few months in.

The first place I went to was ShopStyle, since I had worked there previously and had experienced the problem that Builder aims to solve first hand (non-dev teams needing to build new pages all the time, and devs without time to constantly code up all this new content).

So since I had a good rapport with them and this was a pretty "hair on fire" problem with them, they were willing to be my first customer. I also charged them very little - at first at least ;)

Then I got new customers from word of mouth and from being able to demonstrate what ShopStyle had done (the pages their non-devs were making and how it was helping their business). When people liked the product they were happy to refer me to others who could use it. This was slow but it eventually got me a handful more initial customers, some I knew personally and some I didn't.

Anyway, happy to answer more about the process if you have other Qs or interested in more detail!

Edit: Typo