Carbon, rods welding supply shop or the inside of non-alkaline dry cell batteries (increasingly hard to find these nowadays)
Carbon dioxide, solid "dry ice," useful for low-temperature condensations and (rarely) as a chemical; often found at party supply outlets. I get mine from the nearest Baskin Robbins.
Camphor drugstore
*Caustic soda, found in crystal Drano, and other brands of drain cleaner.
Dextrose drugstore, health food store, even (some) well-stocked grocery stores
Diammonium phosphate - nutrient for wine fermentation. I found mine in the brewing/wine section of the local Thrifway. Hooray for the Northwest, land of DIY alcohol!
Copper sulfate root killer, available at hardware/general stores, usually in the plumbing section; also at some pharmacies.
Citric acid I found mine in the drugstore section of the local Thriftway. But it was expensive - $9.00 for 113 grams! I have since found that citric acid is sold in conjunction with home soap making (search online) for far less. It is also a photo chemical and used in brewing. Some grocery stores may stock it (one alternate name is "sour salt.")
Corn starch baking aisle at grocery store
Diammonium phosphate nutrient for wine fermentation. I found mine in the brewing/winemaking section of the local Thrifway. Hooray for the Northwest, land of DIY alcohol!
Dichloromethane, is found in chemical paint strippers (not the newer, less-toxic ones); it is generally mixed with another chemical or even multiple other chemicals. For some purposes it may be used as-is. For others it will need purification (not covered here). Look in paint store/aisle or hardware store and read the labels on the cans of paint remover.
Ethanol most famous of all alcohols. The stuff people like to drink. You can purchase denatured alcohol as a solvent and fuel in paint stores/aisles, and this is mainly ethanol, but such alcohol also contains a significant percentage of methanol, which may interfere in some reactions. 95% pure ethanol (the remainder is water) can be purchased at liquor stores under a variety of names; the most famous is probably Everclear. You will, of course, need the cooperation of someone 21 or older, and it is pretty expensive due to heavy taxes. It would also be a good idea to explain beforehand to parents why you need this.
Ferric (iron) chloride etchant for printed circuit boards; good electronics store or online (not Circuit City); I've seen it at Fry's Electronics.
Glycerin drugstore
Hexamethylenetetramine, solid fuel tables for portable stoves, barbecues, and campfires; found in sporting goods stores/aisles.
Hexamine see hexamethylenetetramine
Hydrochloric acid sold in paint stores/aisles, hardware stores, pool centers, agricultural centers, as "muriatic acid." As sold it is usually quite concentrated and will emit fumes of hydrogen chloride so it's not a great idea to use it indoors.
Hydrofluoric acid, dilute sold in grocery stores (cleaning aisle) and some hardware stores in brown plastic bottles as Whink rust stain remover. It is quite low concentration as sold; nonetheless, make sure you really do heed the safety warnings on the bottle.
Hydrogen peroxide, dilute grocery stores and drugstores
Hydrogen peroxide, less dilute sold in hardware stores, paint stores as part of "2 part wood bleach" kits or solutions. This hydrogen peroxide is generally between 30% and 40% concentration, as opposed to the 3% antiseptic solution from the drugstore. Thus it is not "concentrated," but it is far more potent than your garden-variety peroxide. Caution is definitely advised. You may be forced to buy 2 bottles of solution together in a kit when you really only want the peroxide bottle. The other bottle usually contains a solution of sodium hydroxide, sodium silicate, or a mixture of the two. Locally, at Portland Paint and Supply, I have found that "Daly's" brand wood bleaching solutions are sold unbundled, so I can just buy as much peroxideas I want without wasting money on the other solution. Solutions of 30% concentration, of very high purity (and no stabilizers! ) can be ordered from a variety of places online. It seems to be used as a sort of home remedy for everything under the sun, so you might check health food stores also. Solutions in this approximate strength range (not as good as wood bleach, but close) can also be found at pool/spa centers. Salon centers sell concentrations between 6% and 12% for bleaching hair, but it is weaker than all of the above sources and fairly expensive too.