In my case, it was from posting to an email discussion list (for Photoshop plugin developers & users), announcing the product & asking people to check it out. I’d already been a member of the email list for a long time & participated there and they had a culture of allowing members to announce things they were working on, so it wasn’t about randomly spamming user groups.
I don’t think I got any sales from the email I sent, but one developer on that list saw it, and he then mentioned my product in his own email newsletter with 30,000 subscribers. So that’s where all my first sales came from.
Prior to that I’d just made a website & hoped that people would magically discover it and buy from it. Obviously, that didn’t work! So there were a few weeks of zero sales before the newsletter thing happened.