Technically that is the only natural source, it can be made industrially with hydrogen sulfide and methanol, both of which are very abundant in the interstellar medium and on gas giants (Neptune has large clouds of hydrogen sulfide). The reaction is catalyzed by aluminum oxide, which also occurs naturally. On a world with an aluminum-rich crust, I could see dimethyl sulfide being produced in volcanoes. It was striking that this planet had thousands times higher concentration of DMS than Earth; maybe it has vastly more algae, maybe it's purely geological.