Thanks for a great question and I am curious how others discover new music. I have Shazam and it will help identify a song I haven't heard before if I am out and about and hear a song I like.
I used Spotify's Discover playlist that updates every Monday and every Friday I listen to all the new music. I will also listen to the new music Friday playlist that is geared towards me based on my songs. Often those songs aren't mainstream because I have a lot of music that hasn't gone mainstream. The more songs you add the better the algorithm is. Also with Spotify, I will listen to what my friends are listening to. Additionally, I will use the "related artists" feature and help build my music library. I was able to add a ton of songs that way. I've discovered a lot of songs on Spotify Community and a British music message board called "Drowned in Sound community. I love how Spotify will also play songs after the end of playlist under their recommended songs. There is usually a song or two I will discover that way. One of the coolest ways to discover new songs is just type in a word on the search bar and just listen to all the songs with that word in it. It works that way with artists. I find that one of the most satisfying discoveries when I find someone that way. Daily mixes are a good way to find new stuff too.
Pandora was one of the very first to create a streaming type of service.
Youtube is one of the best ways to discover new music. Great with recommendations.
I'm not familiar with iTunes but I imagine it's good also.
I will use movies, tv shows, word of mouth, whatever it takes to discover new music. I think putting together folders is very important to categorize your music. I have so much that it's easier to find music. Hope this helps answer your question.