Bicarbonate of soda with its own acid already included, so it needs only moisture and heat to work. That self-contained acid is the whole difference from bicarbonate of soda, and why the two are not interchangeable.
Sourcing
Any supermarket. Buy the smallest tin you will get through — this is one of the few store-cupboard items that genuinely expires.
Varieties
Single-acting, Double-acting, Gluten-free
Quality cues
Test it: a teaspoon in hot water should fizz vigorously and immediately. A lazy fizz means flat cakes.
Substitutions
Bicarbonate of soda plus cream of tartar (1:2)
Storage
Airtight and dry. Never dip a wet spoon into the tin.