By that I mean where they "hangout". How you can reach them.
How? Talk to them. Build relationships. Maybe online if there are specialized forums, or maybe at the bar, or maybe by first visiting them physically. Not with an hidden agenda but with the sincere goal of helping them to solve their problems. Then you will see if your product is a great solution to their problems and if it can leads to a business relationship or if you need to iterate on your MVP.
Selling a new product is all about doing things that don't scale in order to get as much sincere feedback as possible.
Finally learn to not take rejection personally but as feedback.