Looking to get your music online and generating $0.0000001 a stream?
Without getting heavily into the whole business of distros, aggregators, and the whole spaghetti tangle, here are two useful lists:
Spotify's list of preferred distributors: https://artists.spotify.com/directory/distribution
and Apple's list of preferred distributors: https://itunespartner.apple.com/music/partner-search
(thanks to my PRO, APRA-AMCOS. Don't belong to a PRO? [Performing Rights Organisation] You need to!)
Full disclosure: I use CDBaby because (a) they used to sell CDs (!) (b) I always have and (c) they're a once-off cost, not an annual charge.