Most limited supply profession items are recipes or materials used for producing items. Most of these can be found in vendors in main cities or from vendors of specific factions. As you explore the wilds you may come across wondering or exiled merchants (usually they are goblins or gnomes) that have these.
Here are some charts with the locations of some limited items:
Alchemy:

Blacksmithing:

Enchanting:

Engineering:

Jewelcrafting:

Leatherworking:

Tailoring: