(C )Any digital good that is advertised or offered to a person that the seller cannot revoke access to after the transaction, which includes making the digital good available at the time of purchase for permanent offline download to an external storage source to be used without a connection to the internet.

It shouldn’t be that hard, gog.com manages to do it

• GoG
• Itch.io
• BandCamp
• iTunes Music Store (it’s still around but can be hard to find because of Apple Music streaming service)
• Amazon’s music store (but it’s crap quality and they put audio watermarks in it - don’t use them while the two above options exist)
• Comixology before Amazon bought and deleted them
• Occasionally there are websites where you can purchase video content for download, but it’s quite rare. I remember buying anime from such a site before.

Google play music used to offer it as well.