Weak acetic acid (~5%) is sold in grocery stores as white vinegar.
Acetone is found in some nail polish removers and some paint removers. It may sometimes be found labelled as pure acetone.
Borax is sold in solid form as a laundry booster, all-purpose cleaner and sometimes as an insecticide.
boric acid (H3BO3)
Boric acid is sold in pure form as a powder for use as a disinfectant (pharmacy section) or insecticide.
Butane is sold as lighter fluid.
calcium carbonate (CaCO3)
Limestone and calcite are calcium carbonate. Eggshells and seashells are calcium carbonate.
Calcium chloride can be found as a laundry booster or as a road salt or de-icing agent. If you are using the road salt, be sure it is pure calcium chloride and not a mixture of various salts.
Carbon black (amorphous carbon) can be obtained by collecting soot from the complete burning of wood. Graphite is found as pencil 'lead'. Diamonds are pure carbon.
carbon dioxide (CO2)
Dry ice is solid carbon dioxide, which sublimates into carbon dioxide gas. Several chemical reactions evolve carbon dioxide gas, such as the reaction between vinegar and baking soda to form sodium acetate.
Uncoated copper wire (from a hardware store or electronics supply store) is extremely pure elemental copper.