It's erroneous to think of those volcanos on Io as sulphur volcanos. This is an old idea. Io's volcanos are instead somewhat similar to volcanos on the Earth, where the primary product is mafic rock (possibly ultramafic rock in the case of Io), but with lots of volatiles as secondary products.
In the case of volcanos on Earth, those volatiles include water, carbon dioxide, and various gaseous sulfur compounds. Lots of sulfur compounds. The 1783-1784 eruption of Laki in Iceland and the 1815 eruption of Mount Tambora in the Philipines dumped massive quantities of sulfur dioxide into the atmosphere. Both of these eruptions had very significant (but short-lived) impacts on the Earth's climate. Much more severe volcanic eruptions are now believed to be responsible for many of the extinction events, including the biggest of all, the Permian–Triassic extinction event.
In the case of Io, there's no water, and very little carbon dioxide. The volatiles released by Io's volcanos are mostly sulfuric compounds. If Io ever did have water, it lost it long ago. The same is true for carbon dioxide. Water and carbon dioxide are both light and highly volatile. Sulfur dioxide is 45% denser than is CO2, and is significantly less volatile that are either water or CO2. While Io can't quite hold onto its thin atmosphere of sulfur dioxide, the decreased volatility means that a good portion of that vented sulfur dioxide gas condenses and falls to Io's surface, eventually to be recycled into Io's interior.
With regard to where all that sulfur came from, sulphur 32 is the tenth most abundant isotope in the Solar System and in the Galaxy. The reason for this abundance is that 32S is on the alpha ladder, a fusion process that eventually results in the production of 56Ni (which quickly decays into 56Co and then 56Fe) in supernovae. The alpha ladder is why we see so much carbon, oxygen, neon, magnesium, silicon, sulfur, and iron in the universe.