Never tried it myself, but you may want to attend local enterpreneurship meetups, networking events and similar. If you have a passion for your product and it is working (you said it's growing in users, which is nice) I'm pretty sure you can sell it face to face.
I tried this with one of my projects. I ended up getting offers for a few hundred dollars on flippa. I decided to keep it but now it's always in the back of my mind. I think I might just have to shut it down so I can move on.