Currently Wikipedia. I discovered it when I started off with 1970s American composers like Philip Glass, Steve Reich and Jon Gibson. Reading through their entries I fell down a rabbit hole that led to some really great early modular synth musicians like Laurie Spiegel, Mother Mallard's Portable Masterpiece Orchestra and Tonto's Expanding Head Band.
Wikipedia is also where I finally caught up with Robert Wyatt, who I totally fell in love with, particularly Nothing Can Stop Us which has a wonderful cover version of At Last I Am Free by Chic.
It's a great place to find side projects too, like the eponymous album by Klark Kent, an early alter-ego of Stewart Copeland, the drummer from The Police.
In terms of more recent stuff, mostly my Spotify Friends Activity feed. I follow a few people whose taste I really enjoy, and they're constantly coming up with good stuff. I find the auto-generated lists by Spotify are either mediocre - where I feel like I'm just being pandered to - or simply bizarre.
Edit: Matt Sweeney also posts good stuff on Twitter - https://twitter.com/theheavyjamz