You can post an anonymous survey on Reddit or specific Facebook groups, and have people leave their email if they are open to a follow-up interview.
If you already have a prototype, there are paid online services like UserTesting (https://www.usertesting.com/) where you can specify tasks for users to complete and get verbal feedback.
Thinks for the link, I looked at usertesting but it seem for bigger company base on the price
Did you find any explicit mention of pricing ?
Basicaly I'm currently want to ask to more that friends or family for that reason.
They will probably respond on forum but how do I know they are potentiel users ? And how can I continue the conversation, having feedback for each release ?
https://www.reddit.com/r/SideProject/comments/d0xp35/i_made_...
>> continuing conversation/building relationships
When they ask for a feature, build it immediately if it seems like something smart to build. They will think you are a god because people are used to crappy customer service where they don't even get a response for days. So instead of waiting days to respond, build the features in a few days and respond that you built the feature. Then ask them nicely for a review and you will get 5 stars.
> feedback for each release
Ping the people who you know are power users. Also email every single new signup and ask how they like it.
Happy to provide personal feedback on your idea here too.
Don't preface it as "feedback" preface it as, what don't you like about this?