You can subscribe to bubble FONT on the basis of an individual ARTIST. You have to be a subscriber to an ARTIST's bubble to be able to subscribe to the ARTIST's bubble FONT.
Note that you cannot subscribe to an ARTIST's bubble FONT if you are not currently subscribing to the ARTIST's bubble.
(For example, if you have a one-person subscription to ARTIST A's bubble, you can subscribe to the bubble FONT of ARTIST A but not of ARTIST B.)