Soulseek for music downloads. I've been able to find anything on there, including obscure artists in niche genres that even Spotify didn't have.
I used to have Spotify but went through a rough patch and had to cancel all my subscriptions, which forced me to start my collection back up. I made a list of everything I listened to from Spotify to get it started. For the rest I mostly just keep up to date with artists I like and occasionally look for threads discussing the genre or similar artists for recommendations, or I'll let a YouTube mix do it's thing while I'm working and look into anything that stands out.
Triple J (Aussie Radio station) does "like a version" on their YouTube channel where they invite artists to cover another artist's song of their choice and I've found some really good ones through that.
