I think that "the fountain (or: wellsprings) of pure reason" must be understood as a figure of speech, in order to contrast Kant intellectual project regarding the foundation of metaphysics with the "traditional philosophical method" of relying on the authority of ancient philosophers' opinions.
See Kant's project in the Critique of Pure Reason :
The main topic of the Critique of Pure Reason is the possibility of metaphysics, understood in a specific way. Kant defines metaphysics in terms of “the cognitions after which reason might strive independently of all experience,” and his goal in the book is to reach a “decision about the possibility or impossibility of a metaphysics in general, and the determination of its sources, as well as its extent and boundaries, all, however, from principles”. Thus metaphysics for Kant concerns a priori knowledge, or knowledge whose justification does not depend on experience; and he associates a priori knowledge with reason. The project of the Critique is to examine whether, how, and to what extent human reason is capable of a priori knowledge.