From a “social contract” perspective, the source of rights is often seen as relying on the ability to enforce them. This does not necessarily mean that they have to be enforced by violent force, all the time. There are other means of coercion, depending on the situation at hand (e.g., perception control, belief manipulation, persuasion, economic threat or incentive, etc.). Nonetheless, when all the surface factors collapse, force is posited as the deciding factor under this view.
However, there is also the Kantian perspective to be considered. From a “there is no god but man” perspective, the source of a right could be a logical necessity of sorts. For example, the idea that “Thou hast no right but to do thy will,” as in Liber Oz, could be seen as implying that the right to do your will logically emanates from the pure will itself. Especially if we consider the idea that the pure will is identical with one’s true nature.
Now, continuing with a Kantian perspective, and Crowley’s discussion of this topic in the magical theorems presented in Magic in Theory and Practice, to assert one’s right to do one’s will is logically and intrinsically connected to accepting everyone else’s right to do their will. In this sense, the source of rights is the acceptance of the Law in its innermost sense. To assert my own right is to accept the rights of others, and vice versa (per Liber Nu). Of course, it could be argued that “there is no god but man” equates the “god” with one’s true nature. Viewed this way, the inner god is the source of Thelemic rights, but at the same time it can be said that there is no external source for them.