I have an app in the windows store with a few thousand users. When anyone emails me with feedback or an issue, I usually ask if they want to try the beta version (after I've resolved their issue of course). I've gotten about 15-20 part time testers this way, they've been a mixed bag. Some provide pages if detailed feedback, some barely anything.
When I have a decent, bug free mvp of the next version I'm going to fork out for usertesting.com