Potassium sulfate fertilizer
Saltpeter see potassium nitrate
Silver nitrate photo supply or pottery; this is a very expensive chemical, so expensive that's almost worth buying some bullion silver and dissolving it in nitric acid yourself (pure silver is less than $6.00 an ounce and this chemical is more than $20.00/ounce).
Sodium bicarbonate "baking soda," grocery stores
Sodium bisulfate pH lowering chemical (solid) for pools
Sodium borate available as "laundry booster" for washing clothes; also used as flux for torch welding and ceramics
Sodium bromide sold in small packets for initial bromination of spas; also a photo chemical. Either way it's not cheap.
Sodium carbonate "washing soda" used to aid washing of clothes (grocery store, cleaning aisle); also sold as pH raising chemical for pools.
Sodium chloride ordinary table salt; don't get the iodized variety; you may want to recrystallize the salt to purify it of anti-caking agents (or try kosher salt if you can find it).
Sodium hydroxide grocery store or hardware store, sold as Red Devil Lye; small pellets in plastic bottle.
Sodium hypochlorite, solution liquid bleach, sold at grocery stores, or "liquid chlorine" (more concentrated) sold for chlorinating pools; both contain a good deal of sodium hydroxide although the bottle won't list it.
Sucrose this is ordinary white sugar
Sulfur may be available at drugstores as "flowers of sulfur"; sold for agricultural/garden use as dusting sulfur. The dusting sulfur may have up to 10% non-sulfur materials in it.
Sulfuric acid, concentrated when I first discovered this source I practically did a little jig because sulfuric acid is such a commonly used chemical, yet hard to obtain (or so I thought). There are a number of "professional strength" drain opener liquids that contain sulfuric acid. They are not generally found in grocery stores, but should be in any decent hardware store with the plumbing things. Some brands are more concentrated than others, and some contain dyes and extra additives to protect your pipes. I have been pleased with "Rooto" brand; it seems to be concentrated acid with minimal additives. Other brands that are supposedly good include "Crystal Power," "Instant Power," and "Liquid Fire." All of these are easily recognizable because they come in a plastic bottle enclosed in a heavy clear plastic sack. The concentration of these acids is good (not azeotropic, but greater than 90%) but they may contain substantial impurities, because they are made with waste industrial acid (or so I have heard). Oh, and they really do open up drains like a charm.
Sulfuric acid, dilute sulfuric acid of approximately 30% strength is available as replacement battery electrolyte for lead/acid cells. It is found at automotive stores. Acid of 48% strength is available as a photo chemical. Per unit of acid, these are both more expensive than the drain opener, but should also be higher purity (especially the photographic). I have seen 10% sold as pH reducer for pools, but it's hardly worth buying at that concentration.
Tartaric acid I have seen this sold as a wine making supply
Toluene sold as solvent in paint store/department
Trichloroethylene Sold as Sunnyside Carbo-Sol, available at paint stores; may also be sold under other brand names.
Trisodium phosphate hardware store, cleaning aisle of grocery store. TSP is harder to find nowadays because of marine pollution concerns, so check the label to make sure it's not a similarly-named substitute.
Turpentine paint store/aisle, crafts/hobbies; the turpentine that I have smells deliciously of evergreens, but I understand that there are some varieties which smell foul. The natural stuff is harder to find than synthetic substitutes.
Xylene sold as solvent in paint store/department
*Zinc, can be found in hardeware stores as "moss remover". I have found it in two different forms: Pellets, and sheet (used on roofing).