If you are talking about C_Transmog.GetSlotInfo and such I just looked up how Blizzard does things in their code. You can export the code that Blizzard uses for the UI with a console command.
1) On the Battle.net launcher go to Options.
2) Select Game Settings
3) Check "Additional command line arguments" and then type in "-console".
4) Then go to the character selection screen and hit tilda (~).
5) Enter "exportInterfaceFiles code".
All the files will go into folders in "C:\Program Files (x86)\World of Warcraft" or where ever you have the game installed. After that you can remove the console command.
You can also export a lot of the art with "exportInterfaceFiles art". But you can expect it to cause your game to lag out. You will need
BLPView to be able to see the art in Windows Explorer.
Sites like
this one also can help compare how things change from patch to patch.