Any halfway decent chemical supplier should have sodium sulfite. The stuff is a ravenous oxygen scavenger and has lots of industrial and even culinary uses. A weak solution of sodium sulfite can be sprayed on salad bar greens to keep them fresh. Some red wines have a quantity of sulfite as a preservative. I don't think the classifies are the most efficient way to find this stuff, seeing as it is so ubiquitous. Search for a restaurant supply house or chemical supplier in your area and start talking to them.