Sulphur is literally everywhere. Either scan some planets that contain a decent % chance, preferably with some bio or geo sites to make it a little easier to find larger groups of things that might spawn it, or just drop onto the surface anywhere and start driving around shooting rocks.
Alternatively (and IMO, the better way) trade down some higher grade mats. A visit to the crystalline shards sites is where I do all my raw mat shopping, though it's a bit of a trek at 1500 LYs out, but you can fill all your high grade bins to max and then trade some of them down and get everything you'll need.