If you're speaking about the primordial source, then comets and nebula whencefrom the moons of Saturn gradually accreted seem to be natural candidates.

The main problem is a search for explanation of the stark disparity in evolution between Titan and other moons of Saturn, as well as the structurally quite similar satellites of Jupiter. Moreover, all traces of methane in Titan's atmosphere should have been converted into more complex hydrocarbons within 50 million years at most, hence the methane is constantly replenished.

Yet another enigmatic feature is the difference found on Titan between composition in mixtures of atmospherically-derived tholins and ice-formed ones; the latter are having been produced there through radiolysis of clathrates of water and methane or ethane.