I don't think there are any more of these sites around. Back in the day before music streaming, there were subscription sites like napster that gave you a certain number of downloads per month.
To cut it short: I don't think there's "flatrate" or "subscription"-based shops around anymore, they were all replaced by streaming services like Spotify, Amazon Music, Deezer, Tidal etc.
If you're big enough, you might be able to get into a sampling contract with one or more labels, but that's usually reserved for FM/DAB+/Satellite stations.
To cut it short: You can buy songs individually or whole albums/samplers on iTunes and Google Play, but that's usually where digital music shopping ends these days.