Ask HN: Where did your second customer come from?

Tonight I confirmed my first sale for a product that I've been thinking about for a while. The product hasn't been built yet but the customer is keen to collaborate and help me get it off the ground. Obviously the first iteration will be very strongly influenced by this customer's requirements.

I'm keen for this not to be a one-off but I have a limited number of friends whose employers might need this kind of document-generation tool.

If you've bootstrapped a product like this, where did your second customer come from?