What is the generic term for a thing you subscribe to?

Solution 1:

One normally subscribes to a Service - whether that be the provision of a monthly magazine, the the provision of technical support, the provision of access to a particular website, etc..

service
1. [mass noun] The action of helping or doing work for someone.
1.1 [count noun] An act of assistance.
1.2 Assistance or advice given to customers during and after the sale of goods.

Solution 2:

Publisher. It maps nicely to real world subscribers who receive subscriptions to periodicals from publishers. And the general pattern is known as the Publish-subscribe pattern.

Solution 3:

Maybe, it's just Source?

Because it's the source of the 'things' which the subscriber receives.