Iron oxide pottery supply
Iron sulfate pottery supply, some agricultural centers
Isopropyl alcohol grocery store, drugstore; many places stock 99% purity now, so get it in preference to 70% or 91%.
Lithium carbonate - pottery glaze material
Lithium hypochlorite - rare chlorination chemical; likely impossible to find outside of a dedicated pool/spa center or online retailer.
Lead easily found as balls and wire in sporting goods store/department
Lead oxide pottery/ceramics; may be sold as "red lead" or "lead red."
*Magnesium, found in camping stores as "firestarter bar".
Magnesium carbonate drugstore, perhaps some sporting goods stores
Magnesium sulfate "Epsom Salt(s)," often found at grocery store or drugstore; pottery supply; photo chemical; may also be sold as fertilizer.
Manganese dioxide pottery supply, fresh dry cell batteries (they need to be carefully disassembled and the black material removed).
Methanol this is the simplest alcohol of them all. Found as automotive gas-line antifreeze and water remover ("Heet" or similar), also available as photo chemical. If you just need a little bit and don't mind it being highly diluted with ethanol, denatured alcohol works.
Methylene chloride see dichloromethane
*Naptheline, found in mothballs bought in hardeware stores
Nitrogenous matter, organic once in a while books (especially older ones) may call for or reference "nitrogenous organic matter." Basically, this means anything that came from living creatures that has a considerable amount of nitrogen (usually indicates proteins) in it. Blood meal, made from dried animal blood, generally fits the bill. It is available as a fertilizer at many garden/agricultural centers (including ordinary chain stores).
Oxalic acid available as fairly pure crystals in the form of wood bleach powder; found in hardware stores and paint stores/aisles; photo chemical.
Potassium bromide photo supply; for some reason this is much cheaper than the sodium salt.
Potassium carbonate also known as pearl ash, or potash; may be found in some pool/spa pH raising products; used in pottery/ceramics; photo chemical; clean hardwood ashes contain a considerable amount (but for most purposes would want purification).
Potassium chloride found as a sodium-free alternative to salt in materials for softening hard water (found at supermarket, hardware store); beware that it comes in big sacks. May also be found in big sacks as fertilizer. Found in more convenient sizes (and greater unit price) as salt substitute in grocery stores; check label to make sure there is no sodium contained; also photo chemical.
Potassium dichromate pottery, photo supply
Potassium hydroxide, solution the main (only?) ingredient in some drain openers; check the label to be sure it's not sodium-based or an acid opener; found at grocery stores, hardware stores.
Potassium iodide photo chemical, also sold (generally as pills, sometimes pure) as an emergency supply to help prevent the body from absorbing radioactive iodine in case of a nuclear disaster (look at stores, online or off, catering to survival gear).
Potassium nitrate found as fertilizer in large quantities, also as stump remover (found at garden centers) (this is my favorite compromise between price, size, and quality: $3.75 for a 5 pound box with some impurities); found at some drugstores (good purity, also expensive); may raise suspicions due to popularity of making pyrotechnics/smokebombs/black powder). Sold as a ceramic glaze component.
Potassium monopersulfate pool/spa non-chlorine shock treatment
Potassium permanganate sold for manually recharging water iron filters; I found mine at Sears in the plumbing section. I have checked around though, and not all Sears stores have it. So if a local Sears doesn't have it, order it from Sears' web site, or obtain it as a photo chemical (more expensive, though neither is exactly cheap). I believe that it was once found in drugstores, but I have never found it there. This may be another eye-raising chemical, since it can be used for fireworks/explosives and drugs.