We've tried to align the vendors to be as easy to manage as possible. Whereas we understand it can initially be confusing, adding the .additem command comes with it's own set of issues. For example, there are many broken items in the game, and items we simply don't want players to have.
There are however plans to implement the .additem command only including the available vendor items in the upcoming MoP update, which will hopefully eliminate your issue.
Currently we would recommend you look up the item you'd intend to add to yourself via wowpedia or wowhead and look at the item level, then in the mall browse through the vendors and use the item level as a guideline to locate it.
To make things easier in our current state, I'd like to refer you to some helpful websites;
[url="http://forum.wowroleplaygear.com/"><s></s>http://forum.wowroleplaygear.com/
[url="http://www.icy-veins.com/wow/transmogrification-guides"><s></s>http://www.icy-veins.com/wow/transmogrification-guides
There are however plans to implement the .additem command only including the available vendor items in the upcoming MoP update, which will hopefully eliminate your issue.
Currently we would recommend you look up the item you'd intend to add to yourself via wowpedia or wowhead and look at the item level, then in the mall browse through the vendors and use the item level as a guideline to locate it.
To make things easier in our current state, I'd like to refer you to some helpful websites;
[url="http://forum.wowroleplaygear.com/"><s></s>http://forum.wowroleplaygear.com/
[url="http://www.icy-veins.com/wow/transmogrification-guides"><s></s>http://www.icy-veins.com/wow/transmogrification-guides