Here's a very simple trick to get your items or recipes that are limited supplies.
Open the reputations tab, find the faction of the NPC selling your item/recipe, and declare war on that faction.
You can now kill the NPC and when he respawns the limited supplies are back to full. Buy them for yourself or resell.
1) Note that this can only be done with factions that are not major factions.
For the Alliance, you can't do that with vendors from these factions:
Stormwind
Ironforge
Gnomeregan Exiles
Darnassus
Exodar
For the Horde, you can't do that with vendors from these factions:
Orgrimmar
Undercity
Darkspear Trolls
Thunderbluff
Silvermoon City
2) Note that by killing a NPC from a non-major faction, you will decrease your reputation with that faction.
For example killing a Goblin from the Steamwheedle Cartel (That is, from any of the 4 goblin neutral towns: Booty Bay, Gadgetzan, Ratchet, Everlook) will decrease your reputation with not only the NPC's town (for example, Booty Bay for a vendor in Booty Bay) but with the entire Steamwheedle Cartel.