I mean the other textures which are not in game for example in the tutorial quanon uses a chain mail texture and this texture is not present in any game.
Well I often just start with Google.
And use general terms, like: metal texture, leather texture or cloth texture...
Think of many kind of materials, like stainless steel, copper,zinc, marble, silk...
If the added word "texture" doesn't give what I want, I just look for material name.
Else I try these sites:
Some of the better photos you need to pay for :-s , but there still lots of them for free.
http://www.sxc.hu/category/1611)
http://www.sharecg.com/b/6/textures?PSID=14621f6fe1337128f8da4d6c1b0b08b0)
http://www.openfootage.net/?cat=3)
http://www.istockphoto.com/file_search.php?action=file&text=Stone+texture)
Other sites are those like : Photobucket.com, Imageshack.us, ... etc
It takes some time to find them, but sometimes you can use sand as metal.
You would be suprised what some slight recoloring can do.
Edit: Mayang was a good site to, but has been down now for a week or longer... :(